The other day I noticed that several of the podcasts to which I normally subscribe started having cover art visible when I played them in iTunes. I thought that would be fun, so I tried to figure out how I could do this. I could, of course, just load the mp3 file into iTunes and then drag the desired image file onto it, but that seemed like it required more button clicking than I normally would like, so I tried to find a command-line gadget that I could add to the script that I normally run to upload my podcasts to my server.
The winner: eyeD3, a python module that allows you to add id3 tags (including APIC tags which are used to put in artwork). It includes a command-line program, which you can invoke like:
eyeD3 --add-image=foo.jpg:FRONT_COVER some.mp3