How do I add a custom album cover to an .mp3 file?

Ok, so let's say I have a .jpeg file that I want to somehow sync with a particular .mp3 file that I have, so that when I play that song on my iPhone, the .jpeg i coupled it with will appear in the background (essentially making the pic its "album cover").

Is there a way to do this on my own with custom pictures? Or do I have to get the album covers from iTunes and tough noogies for me if they're not available there?