Typically good 500x500 pictures can be right-click saved from Amazon.
Be aware that the "zoom-able" images are not jpeg's
If that isn't good enough Search Google on the "Images" tab.
and include "album art" along with Artist and album title.
Typically there are 1000x1000 or larger images for the art.
Personally I find 500x500 to be good enough as these are actually
slightly larger on my 24" Monitor than the physical CD artwork.
Also I generally only bother with front cover art.