When I want to assign a different type of cover art to an image, other than Front or Back, the program just wouldn't let me. It'll save images classified as Media or Artist for example, only as Front (or may be Back). It's one big TAK files with Embedded cuesheets & 4 embedded JPEG cover arts. The tagging system is APEv2.
And it's not a problem of the audio player (foobar2k in this case), I actually see there is no change in MP3Tag, after I save these covers as Media and other types, it's automatically being saved as Front.