JPG images are frequently very low-quality because of compression artifacts.
It's frustrating trying to replace multiple JPG images at a time!
I have to transfer the image tags over from the JPG to the PNG, upload the new PNG image, change ".JPG" to ".PNG" on the pages that link to the JPG, and then go into the JPG image itself's page itself, and request for it to be deleted.
That process doesn't sound so bad, until you realize that sometimes an image will be linked to from four or more pages!
It'd be phenomenal if there was a feature that would automatically do all of the above for me, or better yet: leave out the image extension in the first place, and have the browser use the image header to determine the file type, like Imgur and other hosting sites do.