I think this is what you are looking for if not let me know!

I create the shape I want my pic on one layer (fill it with whatever color it doesn't matter), then on the layer above it I put my picture. Then I hit control-g and it makes the picture only show up where the shape was. You can then resize the shape or resize the pic and move it around until you get it where you want. This is also how I do a lot of my titles that are filled with patterned paper. First type your text, then add the paper above and then hit control-g and voila its a patterned title. Hope that helps, its one of my FAVORITE things to do with PSE!

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