Hi, (ONE WAY) essentially you going to have three basic layers, the bottom and top layer are going to be the people on the sides and the middle layer the picture of the mystery person. You need to cut out either using your selection tools, or pen tool, mask, etc. the parts of the people you want above the mystery person frame- that layer will be at the top, The full image of the two people at the bottom.
Does that make sense? You could also just cleverly mask the picture layer but the effect will look better with the 3 layers.
/G
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