Basic Editing With Layers
In the Layer Palette, you should see two images in different layers. You can toggle the top layer off and on by clicking the Eye icon to the left of the top layer. To vary the transparency of the top layer so that the bottom photo shows through, select the top layer (by clicking it in the Layer Palette) and then adjust the Opacity control.
You can also change the position of the layers. If you decide that you want the bottom layer on top, just drag it there. There's just one catch: By default, the original bottom layer is locked as the background layer (that's what the padlock icon means). Before you can move it, you must promote the layer. To do that, double-click the bottom layer and then click OK in the New Layer dialog box. Now you can drag the bottom layer above the top layer to switch their position.