Laurels,

Maria did a jam up post on this very issue, Infringing derivative works

To quote her last paragraph which sums things up nicely:

"As a general matter, before AW would get into trouble for permitting a derivative work to be posted here, there are certain legal formalities that have to be satisfied on the part of the complainant. There is supposed to be an opportunity for the site to purge the offending work, if it is indeed an offending work. However, the better part of valor is to head off problems before they arise. When in doubt, ask the originator of a work if you may use it. "