Load 2 images into layers pallette

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.