mp3image is an easy way to write JPEG album covers to mp3 files. It's compatible with ID3v2 tags 2.2 and above (covers virtually all mp3 files). Usage is simple:
mp3image -i image_file.jpg some.mp3 mp3.mp3 files.mp3
The above example will create or overwrite the album art in all the listed .mp3 files with image_file.jpg. It doesn't do back covers or any of the other various images that an ID3v2 tag can hold (it's actually a large list) as most software won't use it when displaying.
mp3image is written in Python and uses the pytagger library, found at http://code.google.com/p/liquidx/
mp3image was written by Jim Wyllie, with some code borrowed from Alastair Tse for his examples on how to use pytagger. Examples cited in source.