Although Apple doesn't return these images, you should be able to retrieve the 225px and 600px versions of the artwork for standard screens, and 450px and 1200px for retina displays. Just replace 100 with the desired resolution. For example:
http://a5.mzstatic.com/us/r1000/040/Music/21/58/3e/dj.bxwywnxk.225x225-75.jpg
http://a5.mzstatic.com/us/r1000/040/Music/21/58/3e/dj.bxwywnxk.450x450-75.jpg
http://a5.mzstatic.com/us/r1000/040/Music/21/58/3e/dj.bxwywnxk.600x600-75.jpg
http://a5.mzstatic.com/us/r1000/040/Music/21/58/3e/dj.bxwywnxk.1200x1200-75.jpg