The great thing about layers in Photoshop is that they allow you to add things and move them about without affecting the original photograph or image. You can try out different shapes and designs on different layers and hide them and make another visible to see which one works best.
You can do this with shapes and other drawing tools such as brushes.
Have a look this video to see how it works.
You can see more Photoshop tutorials at www.jargonfreehelp.com/photoshop-tutorials