It's not anything special. They just have several different images, one of the car and one for each wheel. If you click on one thing - say to change the car color, they change the image of the car to one of that color. If you click on a different wheel type and/or color, they change the wheel type and/or color of the two wheel images.
The way they're doing it is a little more complicated, but that's really all there is to it (in simplest terms):
<a href="#" onclick="document.getElementById('carimage').src = 'redcar.jpg'; return false;">RED</a>