Earth-1024x512.jpg is a resized image from
http://maps.jpl.nasa.gov/ (courtesy NASA/JPL-Caltech)
Image can be freely distributed but should credit
the NASA/JPL-Caltec.

Earth-512x256.jpg is also a Nasa image but was obtained
from Astronomy Picture of the Day,
http://apod.nasa.gov/apod/ap081005.html

brick001.jpg and metal003.gif are from
http://www.grsites.com/archive/textures/
a collection of free background images.

mandelbrot.png is original by David Eck and is 
released by him into the public domain.

I'm not sure where I got marble.jpg, but it was
made available as redistributable.