Abusing this system by willfully replacing posts with incorrect files may lead to disciplinary actions against your account.

Select the file (or insert the url) that you wish to replace the current post with. This preserves all votes, favorites, and comments.
Add a link to where you found this new file in the source field, or explain why you cannot add a source in the reason field.
Put an informative reason in the reason field, such as "Larger PNG from FurAffinity".
Ensure that the file you're submitting is actually better. Larger file size/dimensions, or png extension is not always better. Check for JPEG -> PNG conversions, and artificial upscaling.