User blog comment:Trellar/Screenshots: Tips & Tricks/@comment-227438-20120329202101/@comment-961279-20120330025456

Since the wiki generates a new file when it is displayed at a smaller size instead of sending the full picture and letting the browser do a resize, JPEG loses even more detail at smaller sizes. Our wiki has a comparison page that shows how JPEGs and PNGs handle different kinds of images at the smaller sizes.