I have a question about legal rights. I'm doing a cartoon-based video clip (animated). I wanna use a real picture of a celebrity in it (it will be a poster hung in the singer's room). Think: South Park.
any legal issues?

Yes, there are. Right of privacy / publicity (depending on whether the person is alive or dead), and first amendment parody Huster v. Falwell type rights. You should consult an entertainment lawyer.

The other issue is the copyright on the photo. You have to acquire the rights to it unless it is in the public domain already.

Yeah good point. :) But finding legal photos from somewhere, even that you might have to buy, shouldn't be too difficult.

for celeb pix you can buy try wireimage.com

good luck.