Having never taken a photoshop class (or art class ) I never learned how to do this. The question is how do i take a picture and map it onto and object and still retain the objects textures or reflections.

An example i'm going to use is Marcobalistic's recent car image where he had the gheto paintings along the car and even the door handle while still keeping all the original reflections on the car?

Also, How do you take the textures (of lets say a ship) and put it onto an object and still have the ships granular effects ect.?

well see if this helps you. It's in the tutorials section. I think this is what you mean.
http://photoshopcontest.com/tutorials/7/blending-layer-modes.html

MG pointed you to a good spot. You should also look up displacement mapping tutorials. Usually those are tuts for doing things like tattoos but the techniques still apply to what you want to do.