Yep, the ASIN tag works well for CD's, DVD's and books, and many other "hard" items, but for some reason, Amazon uses a different coding system to display MP3 images. Why? Who knows.
I will look into this and see if I can tweak it.
At worst, what I could do is put an "Image not found" icon in place for those items without an image, but I will explore the Amazon image system and see if I can do better.
cheers
Rob