You don't, you make sure you get it right from the start!
"this was shot live. It took 26 tries to get it over five hours"
My question is, how do you throw cats to make em look like on the canvas, or rather actually, how do you throw cats, the canvas was painted in afterwards on the photo (look at the shadow from the easel on the floor...).
EDIT: Interesting, the picture on my flickr link, has a painting in it, but the one in this thread don't, anyone?
EDIT2: Here's the explanation