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It's fairly straight forward Barb.
Open the two images in PS
Select the mover tool (V Key)
Drag one image onto the top of the other. (If you hold down the Shift key at the same time the dragged image will automatically be centered)
In the layers box select the blending option as "difference"
Now use your move mouse or arrow keys to move the top layer until the whole image turns black or as close to as possible. Once the two images line up in difference blending mode the top object automatically subtracts from the bottom object.
Once aligned select normal again.
Done
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