User blog comment:DaNASCAT/Technical Update: April 3, 2012/@comment-3279487-20120403211531/@comment-26686429-20120404103748

If the thumbnail doesn't update, try re-making the thumbnail with one pixel's difference (fixes it in this case).

Still, an annoying bug, and doesn't fix the fact that clicking through the file page gives the old image, this could be related to the caching issues that are going on at the moment, which Tim mentioned should be fixed in 24-36 hours from this post