Open picture file A. This will be the background layer. Go to File>Place and point to picture B, then commit with the green checkmark. B will come in on a separate layer. You can
move it and resize with the corner handles.
If you wish to restrict to an object on B, use one of the selection tools, e.g. selection brush, lasso tool, to select the object, then go to Edit>copy to put it on the clipboard, then Edit paste. It will show up in a layer above A.