Tracker is the program which indexes all the media on the N900. For MP3s it copies the cover art to a filename with the form /home/user/.cache/media-art/album-$(printf ' ' | md5sum)-$(printf '<album-name-in-lowercase>' | md5sum).jpeg. It follows the GNOME Media Art Storage Spec, except that it always uses a space instead of the album artist. (That's been reported as Bug 7263. I've tried manually copying an album cover to this location for one of my Ogg Vorbis albums, and it showed up in the media player like it should. So for me the next step is to modify the tracker-extractor-vorbis package to copy the album covers like the MP3 extractor does. (Bug 7413)