The header image rotates, and is made up of interesting images that I have found during my wanderings around the internet. All of these pictures are either licensed under the creative commons (or similar) license.
NASA
The majority of the images, are taken from the NASA image library:
NASA has a pretty cool policy regarding using their images. Although not creative commons, it is fine to use them, as long as long as it is not for commercial use.
Others
The original image is by Ian Hampton and can be found here
