the only problem is doing a thing called keystoning. It´s when you have a 2d object but basically "tilt" it on the z axis as if it were a 3d object. I haven´t figured out a way to do it in photoshop or gimp, but if somebody else knows, I´d love to hear it.
as you see in relli´s pic, his car was perpendicular to the viewer, and the pic of the rims he used lines right up, so with a little careful editing he was able to make it look realistic. However, if the picture of his car had been taken at an angle, (say a 3/4 view), then the pics of the rims he would have used would have been incorrect. the perspective would have been wrong. The wheels would be viewed dead-on and the car is at an angle, if you dig what I mean...
anyway, if anybody knows how to do keystone transformations in photoshop, post it up because I personally don´t know.