User blog comment:Trellar/Screenshots: Tips & Tricks/@comment-1603243-20120420034655/@comment-957747-20120420050005

GIF is also a good alternative, not just PNG. JP[E]G is the only lossy format of todays popular image formats. There's a good read on JPG's lossy format here for those interested.

In short, JPG shouldn't be used for images that would want to retain quality over a period of time... even virus scanners that actively scan files on your computer can drastically degrade a JPG image in a matter of months.