So, you want to find a good image for a page. The image is a great attention-grabber; it's likely to be the first thing a person sees when the page loads. In a page about the work, a good image will introduce the work; in a trope page, it will illustrate the trope and help the reader understand it. Just like a good name, a good image follows the mantra "clear, concise, and witty" (in descending order of importance: clarity is key, the other two are decoration). There are a lot of good ways to go about making sure our images are clear, concise, and witty... let's look at them.
Image Pickin' is fairly good at picking images. If an image was chosen by Image Pickin' and you think it should be changed, don't just unilaterally replace it; start a new thread. The Image Pickin' crowd usually leaves a commented-out note about this in the page source.
When you make a thread, be sure to use the actual location of the trope or work. The wiki won't tag the article if you use a redirect or don't spell it correctly.
The most common picture for a works page is a title screen or box art. It's what most people will see when they look in the store for a copy, and images are fairly easy to come across.
Official promotional art such as movie posters are common, including cast pictures. This is particularly common when the actual cover is a flat color and a logo or something similarly minimalist.
Unlike many other types of work, Fan Fiction, Webcomics, and Web Original have relatively easy-to-access creators. If you want to know what they'd prefer, send them a simple email with links to both the site and the page in question. For instance, this was how the page images for xkcd and Darths & Droids were chosen.
Quality of image
Some images simply have bad image quality (compression artifacts, pixelation, and so on). This detracts from what the image is showing, and simple quality upgrades are something you don't even need to ask to replace. For example: Giant Enemy Crab had this image◊, but we replaced it with this higher-quality version of the same thing◊. This is a free action: You don't need discussion or approval to swap in a higher-quality version of the previous image, you can just do it yourself and be on your way.
Size limits do exist. The maximum width is set to 350 pixels, both on the wiki and in the forums. If you need to shrink an image to this size, it may not be a good image anymore (either due to introducing compression/resize artifacts, or by just the details becoming too small to make out). There is no maximum height, but if it has to be much taller than 350 or so pixels to be clear, it may not be a good image.
If you need to resize an image and don't like working with MS paintnote (which has a poor resize algorithm anyway) or other programs, you can use pic resize to do it.
The same goes for watermarks of ownership; attempting to remove them is too much work (and, as a Rules Lawyer points out, against the DMCA). Note that "station bugs" (watermark-like logos in the corner, added by a broadcasting TV station) are not the same as an artist/owner's watermark, though pictures still look nicer without them.
"Artist scribbles" and signatures are fine.
Taking screenshots or scans for illustrative purposes on this Creative Commons wiki falls under fair use.
Real Life pictures: although This Wiki documents devices in fiction, sometimes a real life picture is the best available (Schmuck Bait is an example).
Having the artist's permission is always a Good Thing. If you want to use an artist's work as a page image, it is common courtesy to send an email to the artist and ask for their permission (first!). Here is an example that was sent and responded to with permission:
I am seeking permission to use this picture of yours for the page image of the trope, Villain Decay. If this is alright, please respond to the email or comment in the discussion here. If you wish us to not use your image, it will be changed, if you give permission but want a specific link please respond.
Original art (from deviantART, Flickr, etc.) may be Creative Commons licensed. If you see that, go ahead and use the picture but be sure to follow the CC terms, especially BY (give the artist credit — a link back to their website in the caption generally suffices and is a good thing to do anyway) and Remix (if it doesn't have this it means you aren't supposed to alter the image; resizing is probably OK). NC (non-commercial) and SA (share-alike) are covered by the wiki's CC license.
Which tropes don't have images?: There is a tool that lists tropes without images - see Tropes Needing Images.
Visual Aid: Unlike Works, page images for trope pages are there to help explain the trope, not just provide an example above the example line.
Why are Comics, Manga, and Web Comics so popular?: Because the addition of Speech Bubble allows the image to include Dialogue on top of visualizations, something which multimedia works lack. This, combined with a distinct lack of Motion Blur, leans toward a dominance of these images.
The image is not an example entry: the part of the page above the "examples" line is for explanation and related tropes, the part below is for examples. The image is above this line, unless it's too tall. Being the best/earliestexample is not the same as conveying the trope to other people. The goal of an image is to Show, Don't Tell.
Captions can help: A picture can need a caption to make the final leap, as long as it tells the story right up to the point where the caption closes the gap.
Memes: They're verboten unless, of course, the page is actually about the meme in question.
How can I tell if it's a good picture if I'm very familiar with the work? Ask yourself "If I had no exposure to the source work, would this image still make sense to me?" If you're not sure about the answer, go ask in Image Pickin', there are bound to be a few people who wouldn't know it from a hole in the ground who can help put it in perspective.
A composite picture: Sometimes an image consisting of several examples of this trope shows how a common trope manifests in different works. This works better with tropes that are in Spectacle category and some of the video game tropes. Note and labelnote markers can be used to list the works the examples are from.
Synergy between caption and image where they work together very well to show what's going on. The proper balance is commonly that the picture (being the first thing seen) works as the set up to a joke, with the caption functioning as the punchline.
Quotes from the same work or instance tend to make good captions.
Snark, while not discouraged, is a good second to a meaningful quote.
Things to avoid
Just a Face and a Caption: The visual equivalent of a Zero-Context Example. Most of the time, they only make sense if you're already familiar with the work and the trope. Read the link for more details about why this is a bad idea.
Heavy stylized drawing styles. If a picture is heavily stylized (and not to illustrate a trope about that style in question), it's harder for people to decipher what is going on in there.
Wall of Text. If the image is nothing but a paragraph of text with a character drawn so that there's somewhere to stick a Speech Bubble, it's not a great image ... but it might make a good quote though.
"If you read the example, this makes perfect sense" is not a good way to explain the trope (see the first point).
As stated before, meme-based pics of any kind aren't welcome.
Entry Pimping. The image is not there solely to bring your favorite work to the attention of others.note Of course if your favorite work actually does have a better picture for a trope, that's a different matter...
Spoilers. There's no code for spoiler-tagging an image for people who don't want to see things spoiled. And once you've seen a spoiler, you can never un-see it. Images tend to draw peoples' attention (they are even shown on our Google search results), Most peoples' eyes are immediately drawn to images anyway
Images based on a Literal-Minded interpretation of the trope's title can be misleading. For example, Pet the Dog is not about petting dogs, it's a metaphor, which is a reason that the page features a kitten instead.
Captions are not mandatory. If a caption text only says "The Trope Namer", "X shows us how it's done", "One of these things is not like the others", etc, this is actually worse than having no caption at all (compare first point).
Not Picturable. Sometimes a trope is too text- or plot- dependent for an image to work, and the better option is to just leave the page imageless than put an image that would confuse the reader even further. Don't fret, it happens.
Most images don't have the title of the work in them — which is a problem, since people will likely want to know what work an image is from. Fortunately, we have ways of getting around this — the most common method is to simply wrap the image itself in a Pot Hole link, making the image clickable. See Text Formatting Rules for how to do this.
Multisource works. When an image is a parody of another, such as a gaming webcomic or something, the preferred potholing method is the image pointing to the parody source, and the caption pointing to the parodied work. If there's multiple source works, a note or labelnote with a list of the characters named and potholed is preferred. Most Common Super Power is an example.
Non-potholed images that you've found can be potholed to the appropriate work if you know it.
If a Real Life image is chosen, you can either pothole to Real Life or don't pothole it at all.
If an image is not potholed or sourced in some way, bring it up in this thread.
Do Not pothole an image to an outside site, the "external link" widget◊ messes with the code and looks funny hanging off an image. Put the outside link in the caption if needs to be there.
When multiple good images are proposed for a trope with no consensus on which one should be used, create a crowner by copying this URL, replacing the "InsertTropeNameHere" in the url with the name of the trope, then pasting it in a new browser tab. Then click "Yes, do it" when asked to create the crowner. Link to it in the Image Pickin' thread, and holler for a mod to attach the crowner to the thread.
The purpose of the crowner is to decide between multiple images that are all equally illustrative of the trope. If an image has been determined not to illustrate the trope (or to illustrate the wrong trope), don't put it on the crowner.
Crowners are generally left open until voting slows down. All crowners must be open for voting for at least three "business days" — i.e. if the third day falls during a weekend, the crowner is left open until Monday, as forum traffic tends to slow during the weekend.