If you wish to become expert at creating photomontages--aka collages--from stills and a variety of artwork, which necessarily entails becoming expert at making selections and at using layers, then here is a book that I can highly recommend: How to Cheat in Photoshop Elements: Create stunning photomontages on a budget by Asch and Caplin.*
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*I have the one for 7 (Focal Press 2009) but there is also one for PSE9 and there may be one in the works for 10 (the clapping hands are to encourage such an encore).