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
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.