Timber Wolves im gonna use an example. First you go to the bodyshop. then click Upload Your Images. Save this image. Then when you choose the picture, search what you named the image. It will be with you the whole time!
In order to color it you use a specific colorin program. It can be GIMP, Pixlr, MS Paint, Etc. Depending on if it has a transparent layer or not. You will see a button in the tools selection saying 'normal' click it, and click 'multiply'. Than just use any color or painting object. When finished, you have to save it as a PNG file, so make sure it is set as PNG. That's usually it.