By selecting the background node (with the
Edit tool)and dragging in a thumbnail image you are able to fill the background with a photograph.
One of the more effective techniques is to bring in a photograph and then use the
Opacity tool to lighten it to give a subtle backdrop to your image nodes. You will use the slider that appears when the
Opacity tool is activated to lighten your background to your taste.
Here is how this montage would look with a color photograph as a background: