I do this with my students every year and get good results. I select 8 photos- some stock, some that I have taken, and have the students create a photo using 5 of the photos. This is a culminating project where students are expected to use all of the Photoshop skills that they have used during the year. I insist that students make at least three different versions and save them using Layer Comps. I have the students turn-in .psd files which I check for the layer comps, layers, etc. The download contains instructions, rubrica list of vocabulary that students are expected to know, and several of the excellent Adobe .pdf "how to" files.