Not really a PS master, I use gimp all the time, but I may have some other recommendation for you: Use decals and textures. Rust, scratches, dirt, whatever happens to cars. Make 'em look less out-of-the-factory if you drifted them, for example. You can add that by photoshop or any other image editing software, or (shameless self-advertisement) use THIS to buy cars and take parts from the used cars and add them to the car to make it look like it just came out of a dusty barn or relatively shady used-car dealer's lot
