Open the image you want to paste by dragging it to the Photoshop icon.
Click and hold on the name tab of the photo you want to paste, a rock in this example. Drag it slightly downward and to the right so that you see both documents.
Be sure you are on the Move Tool and that Auto Select>Layer is selected.
Click, hold and drag the new image into the original one.
Or..........
Click hold and drag the new image from its layers palette into the original document
