Famous persons photo in my indie film

I got a indie feature film coming up for the Aust film festival and there is a scene that i want to do where the main actor is photo shopped into a photo with a famous person. Question is am i legally able to do that without getting the permission of that person because i seen matt stone and tray parker do it on south park and some of their movies putting in the credits that the actor/actress did not give permission

Thanks

Damien

Would your useage be considered satire or parody? If so... good.

If not... is the original image in the public domain? If so... good. If not, then you might need the permission of whomever owns the copyright for the image. (It's not necessarily the person in the image, btw. Quite often it's the photographer)

Also... this is basically it for the USA. I've no idea if AU law is any different on the matter.

yeah it would be a parody. can you recommend any sites that would offer this for free. I might add it to a few US films fest so i need it to be good all round