How do I stop image quality from dropping when I upload an image?

Also, how do I control how image sizing in upload?

I've tried converting them to other file types, compressing them, resizing them, but I'm struggling.

It’s best to resize it to as close to wikisize (350 pixel width) as possible before uploading. Also, save it as a PNG to prevent artifacting.

You do not have control over image upload size. Wiki images are capped at 350p and are forcibly resized automatically.

You can upload images using the uploader for forum posts, it works differently as it doesn't resize images during the upload, thus you can keep the original image, but they will be shrunk when used in articles.

That’s the best route to take. If I’m going to be putting an image on a page, I use Paint.net to resize it to 350 px and save it as a PNG before I upload it. I haven’t noticed very much quality loss doing that.