JPG is alive and kicking, and PNG is overused. Proper compression of a JPG produces not difference from a PNG to the naked eye when done correctly. Then again, Miro is having a lot of problems doing "correct" the days.
As to the 21st century, you mindnumbing binarry "either/or" thinking belongs in the 18th century. Go there and get out of the way until you have some real answers.
The PNG thumbnails Miro generates in the extracted icon cache are unoptimized (A Miro 38.KB PNG becomes a 9.38 KB JPG). Also this icon cache keeps all thumbnails, even very old ones.
There should be a "Clean Miro Thumbnail Cache" option that deletes all thumbnails that are no longer listed in the Miro library. :D