Here is a great Free Keynote Theme I like to call From the Earth to the Moon. This great looking theme features a high altitude photograph of both the surface of the earth and also the moon. It’s pretty extraordinary.
To add the image to an additional slide simple create a new slide and select an image background. The earth and moon shot will be available to you to use.
The default text fields are black, which does not show up on the dark background. These Should be changed to some other color for better readability.