Not sure that user icons were always .png's ... uploads would accept .jpg's and then images would be appropriately sized ... one for profile and one used as a smaller version in forums. Something like that ... so the rev thang you are seeing might be related.
Sorry ... don't have a fix for ya! :| ... don't have a site where that issue has been reported (not that sites don't have problems ... users just haven't discovered and reported that particular issue.).