You can set an image to be displayed in the background, behind the whole pedigree. This is typically used to display a faded, or watermark, image.
To set the watermark, right-click (Mac: use Control + Click) over the pedigree and select Background Settings from the popup menu. Then click the Add Photo button.
You should generally use a lightly colored image for the watermark, else it will tend to obliterate the pedigree drawn on top. The Brightness and Gamma settings (also in the Background Settings window) can be helpful here. The 'gamma' is a way to adjust the relative proportions of bright and dark areas in the photo.
The background image will, by default, be scaled so that it covers the entire page (within the margins) whilst retaining aspect ratio. In practice what this means is that, unless your background image has the same aspect ratio - i.e. it has the same 'shape' - then parts of the image will be cropped.
Sometimes this is not the behaviour you would like. In this case you can change the Mode within the photo positioning settings. This can be found on the Layout tab of the Background Settings window.
Alternatively, use a photo editor to adjust the aspect ratio of your image - by cropping it so that it has approximately the same aspect ratio as the page.
NOTE: As far as PedPal is concerned, the background of the pedigree is just another 'box'. It is just special in that it (a) covers the whole page, and (b) it lies 'behind' all the other boxes. But because it is a box it can have its own photo (which is how the watermark is set) but can also have text if you so wanted it. See Pedigree Design - Box Settings, Pedigree Design - Text, Pedigree Design - Text Style, Pedigree Design - Photos, Pedigree Design - Photo Cropping and Pedigree Design - Photos With Text.