As original this Mini cooper S:

Choose a nice image ( search www.google.com for a suitable image( search for dragons, japanese symbols,...), move it, and put it on the right place:

Go to image->adjustments->hue/saturation and give it the correct color:

Now set the blending option on overlay( in dutch version it's called "bedekken"):

Now pick the pollygonnal lasso tool and select the parts you don't need:

Edit->cut those parts and you're finished:


BTW: This was the dragon I used.