Recently I needed to extract the album art from an MP3 file and came across a really easy to use command line utility called eyeD3 to do just that (among other things). Here is how you can extract all of the album art from a file MyFile.mp3 into a directory called Output.

1) Install eyeD3

sudo apt-get install eyeD3

2) Extract all embedded album art from the file

eyeD3 --write-images=Output/ MyFile.mp3

Pretty simple!