It would be next to impossible to do.

Basically to embed a virus in a .jpg or .gif file, you'd have to change the file extension or add on to it. The servers that handle these pictures 1) scan for viruses on a regular basis (if they are any good) and 2) they check the file integrity (extension, any known virus extensions, etc) before they allow an upload to remain on the server.

D'elTarra