WORKING WITH LAYERS

1.  Enter Edit mode; open the Layers Palette (F8) and the Materials palette (F6).

2. Open main image and duplicate it (> Layers > Duplicate).  When adding new layers to the project,
select the project and then paste the image to be added as a new layer. 

4. Select and copy image to be used in background and place the image "beneath" the main image;
select the "Move" tool (arrow) to enlarge and/or move the background image.

5. Select and copy image to be used in the foreground and place it "beneath" the main image.

6.  Use the Eraser to erase the unwanted foreground. 

--Erasing part of the main image to show the background.  [See settings for using the Magic Wand below;]

Helpful tutorial: https://www.youtube.com/watch?v=6qNJC897zJg .

--Settings for the eraser too (below).