I am sure by now, all of you are quite conversant with the Nikon D90's post "Image Overlay" capability but its more of a post processing work you are doing here. If you really want, the fact is that you can do it on-the-fly with the Nikon D90's Multiple Exposure feature (in shooting menu)! Turn it on and shoot up to 3 images before the camera auto-layered them into one image for you. Talk about creative possibility here...another cool feature I discover today.
Now, based on the manual, this intended feature is to enrich one single shot (preferably on a tripod) with its auto-gain engine. Sort of moving into the HDR arena but not quite. But results show that it does give the final result an added boost in color and tonal spatial gradients. Some folks use software like Photoshop to do this but I must say its cool to be able to do it right with our Nikon D90!