So...did you convert the .art files to gif and then post them? Or did you just rename them?
no, that's the problem. when an AOL user who has the 'accelerate download of images' feature turned on in their AOL setup, all of the images in the web pages they view are compressed into ART files by AOLs servers before downloading. the extension of the files isn't changed. (in fact, there's no obvious indication that the files are not really JPEG or GIF except that they suffer a bit in quality. ART is lossy compression.) when this AOL user does a 'right-click save image', the image gets saved as is, so the user thinks they're saving a JPEG or GIF, but they're not. since they have AOL on their computer, they never notice. it's only when they email the image to friends, or upload the image somewhere that there are problems.
i've looked about, and it seems to be pratically impossible to convert the ART format back to real JPEG or GIF unless you have the AOL software installed (or other $$ software). even if you use IE and save the image as BMP it will still really be an ART file with a .bmp extension on your harddrive...
i'll probably eventually tweak the avatar upload code to check the magic numbers in the file instead of relying on just the extension to prevent accidental upload of ART images.