Do you mean something like a stack of photographs, where each photo still has its rectangular border -- or a montage, where you cut out sections of photos and compose them together (like the one with the caption, "montage of pictures from Tom's childhood": ) ?
If you are referring to blending them together..Drop the opacity of the top picture over the other and blend it in..You can also use a SHape for the top image and change the opacity..
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