Select the object (ie the wireframe is red) and render your scene as normal and save image. Then go to render options and select mask render, then render again and save image again under a different name (maybe with mask added to it). The mask render will give you a black (everything but the object) and white (the object) render that you can use in photoshop. Someone else will have to explain how to do that since I don't have photoshop.