Copyleft defined: I grant you permission to use this image however you like, provided only that you grant the same permission to anyone who chooses to use the image (or its derivation) from your website.
So, technically, taking an image provided under the copyleft rubric, and offering it for sale, or otherwise restricting its use is a breach of copyleft. The degree to which an image is modified and becomes an original work, is a very gray area of intellectual property law.
I like the idea that I could protect an image I made -- either from being used without permission or from having it's use restricted by monetary issues. Of course you can argue philosophical and moral advantages of both approaches -- I think there is value in both, a "time for every purpose under heaven."