Use your own personal judgement.
Is the original image a BAD image? Is it Just A Face And A Caption, or is it filled with words, or does it completely rely upon text to convey meaning instead of the picture? Then I'd host an image thread, unless you have an obviously good choice to replace the image (Note: By obviously good, I mean the image must fit the trope, not the example). Better safe than sorry.
Is the picture fine? Then tread carefully.
Do you have a better quality version of the same picture? Replace it in a flash.
And if someone else changes the picture? Well, use your own judgement to decide if the change was for the better or not.
Fight. Struggle. Endure. Suffer. LIVE.Only do it if you trust yourself with the decision. I don't trust myself with it, and I don't really trust others, as I've seen too many good images taken down with the line "my picture is a much more triumphant example". Or "I hate that series".
Higher rez pictures or nearly identical with mild improvements are something I do on occasion.
edited 29th Dec '10 3:00:20 AM by Deboss
Fight smart, not fair.One thing I do to preserve good old pics is put the former pic in an (sometimes brand new) Image Links page for posterity. While this is already value added to the page, it also works as a form of insurance should the replacemen become controversial.
In general though, I agree with Deboss that there some image changes are done with pretty flimsy reasons. In those cases I bring it up in Image Pickin, which is especially useful if you aren't sure you're unbiased (which may be the case if it's YOUR pic that was replaced) and can't think of a good replacement. Worse case scenario? It goes pictureless.
The one thing that especially sets me off is when someone unilaterally replaces an image that itself was chosen after a big Image Pickin' thread. I always revert those with a pointer to Image Pickin'.
Jet-a-Reeno!
I do that whenever I post a pic from an Image Pickin' discussion.
The only times I ever change or add pics without discussion is if there's no picture on the page already or if the old picture is blatantly terrible. If I'm not sure, I make a thread; better safe than sorry.
Reaction Image RepositoryI said "more triumphant". Being a triumphant example does not make a good image. The last one I remember was Magical Warrior Girl where the image was picked by the forum, had a little girl using a magic staff in an action pose, somebody came by and replaced it with a cast picture from some other series with the line "[work] really isn't an example, replaced with better example".
Fight smart, not fair.

Simple question triggered by a specific example. Did not want to ask this in Image Pickin' since doing so would put up that fine Image Pickin' banner that may not be necessary depending on the answer to this question. Have seen different answers to this, and wanted a single once-and-for-all answer.
Related question: I see someone unilaterally change an image. What, if anything, should I do? (Note that in this particular case, I agree wholeheartedly with it - and in fact was going to suggest it - but I realize such things can't be decided by my personal preferences.)