The shell itself is a multi-replication of a torus. The key here is the texture of the shell: make sure it's set to object space and not world space in order to give it the repeated pattern. The iguana's head was a picture of our iguana taken in a close-up of our webcam. Apply the picture to a terrain. With Photoshop, we created an alpha channel with white where the head was and black everywhere else. This way, you get a little 2-D image that looks remarkably 3-D! If you want to go the extra mile, you could make your alpha channel shades of grey to truely create a 3-D image.