I uploaded a new version of an image, yet the thumbnail used in articles reflects the old version. Oddly enough, this is login-dependent. If I'm not logged in, I see the new thumbnail. If I log in, I see the old one. I can go back and forth. I have cleared my browser cache and reproduced this many times over the past week. It may be difficult for others to reproduce, however, if they haven't seen the old version while logged in, if indeed somehow thumbnail cache is tied to accounts.
Thus, this is potentially related to bugs 16301 and 22593, which were marked as 'workforme'. Please try to reproduce by uploading an image, inserting a thumbnail into a page, then upload new version of image and check on thumbnail.
My particular case is detailed here: