I did this once. Its quite involved and a screenshot might be easier. But I wanted to try to get the original so...
Paste your itunes (http not itms) link into safari and hit enter. Hit esc just before it redirects you to itunes.
Right click the image in the upper left and say "inspect element" This will reveal the html code with your image as a source.
Thats how I got:
http://a1.phobos.apple.com/us/r1000/...175x175-75.jpg
edit: Just found this out. To extract your original (in case you lost it somehow)
Edit the 175 part to 512
http://a1.phobos.apple.com/us/r1000/...512x512-75.jpg

:-)