Photobooth Part

PhotoBooth background won't work?

I have a Mac, and there is the program PhotoBooth, which basically takes pictures of you. It gives you an option to select backgrounds such as clouds, dots, fish, etc., then tells you to step out of the frame. I do, but when I step back in after the background has been detected, the background gets all splotchy, with parts of my room and the selected background showing. Help? What can I do to get the backgrounds to work?

the wall behind you must be clean (with no pictures or things on it)
I have got a mac too.