When copying between photos, drag the selection from the active image window into the other image window. A border highlights the image window when you can drop the selection into it.
Hold down the Option key while dragging the selection to each new location.
To offset the duplicate by 1 pixel, hold down the Option key, and press an arrow key. (This moves the pixels and copies the pixels, creating a blur effect.)
To offset the duplicate by 10 pixels, press Option+Shift and press an arrow key. (This moves the pixels rather than copies them.)