Try this. Open both images side by side then click on the layer tab to bring it down. Decide which image you wish to use as the background and click on the other one to make it active - it should now be the only one visible in the layers palette. Drag the active image from the palette to the background image in the workspace, this will now give you background and layer 1. Use the eraser to get rid of the parts of layer 1 you don't want. You can also delete the original of layer 1 which is still in the workspace. Hope this is of use to you.