When it comes to selling a derivative work, I'd consult a copyright lawyer to be sure. But in my reading of copyright law, making a painting of a copyrighted item may create a new copyrighted item belonging to you -- think of Andy Warhol type work. However, there's also the recent case of the artist who made the 'hope' poster of a photo of Barak Obama during the 2008 election. The photo was from an AP news story, & the poster artist did not get permission to use it. While he didn't sell his poster, he did get into legal trouble later on.

Point being that sometimes the rules aren't black or white. Tread carefully!
--Trystan