No computer will come with Photoshop already loaded unless you have purchased it and had the retailer install it for you. Photoshop is an expensive professional level software program.
Snow Leopard comes with Preview installed. It's a basic photo editing program. Here's an article about it:
Preview's Photo Editing Tools Could Be All You Need
Or you could buy Photoshop Elements, which is a not so expensive version of Photoshop that serves most people's needs pretty well.
I have no experience with Preview so can't talk about it with any authority. I use Photoshop on my Mac.