When you upload a picture with the same file name old cached thumbnails don't get updated and you have to manually delete the directory .evocache with FTP.
It also useful when you copy media directory to the local drive, you don't need those .evocache folders
OMG I didn't realize this flaw. I've replaced non-image files many times without issue, and therefore didn't know image replacement did not replace the (poorly planned) file structure for image resizes.
This belongs in the bugs forum!
(As does the lame file structure created for resizes. whoops - that one is already there!)