You can clone from one picture to another if both images are open on your desktop. This is an extremely useful technique. For example, for years I used models of dinosaurs in composite work and I settled for the fairly realistic looking skin that comprised expensive models (these are not toys). Two of the large dinosaur models I bought were actualluy airbrushed by an artist to make them look more 'reptilian'. However, the picture above looks much, much better than anything I'd done previously. I had a photo of a chameleon, and I opened that shot plus the dinosaur photo so both were visible on my desktop. By cloning from the chameleon to the plastic model, I created what very well could have been the skin of this Jurassic-era amargasaurus. I selected the model using the pen tool so when I cloned the skin, the selection (i.e. the marching ants) constrained the skin to within the selection, thus making the process fast and easy.