This would be known as Derivative Art or Fan Art.  Refer to this link in wiki for a brief summary of this topic:

http://en.wikipedia.org/wiki/Fan_art

This second link questions the legality of Fan Art as it pertains to featuring and selling mentioned art at public conventions:

http://chrisoatley.com/fan-art/

One of many such discussions available on the internet.  Of course, this is derailing the question the OP posted earlier, but, as a sidenote, I would recommend anyone considering using existing celebrity likenesses to conduct a web search on the legality of copyright rules as established in the area you reside.