The NASA/JPL Photojournal is an excellent source of material for cover art, web sites and general image fun.

NASA/JPL offers its images under a license that requires attribution if used commercially. Be sure to read this page carefully because there are some caveats. Generally speaking, most images are open for public use.

Have fun!

Image Policy: http://www.jpl.nasa.gov/imagepolicy/

If you are looking for free, open source image editing, you can check out:

GIMP: Professional image editing and a close competitor of Photoshop

Inkscape: Vector graphics drawing program. I used this to lay out and create the book cover or the Kindle Edition of my first novel. It has great text effects.

Scribus: This can be used to format your book and book cover for createspace. I am still climbing the learning curve on this one, but I will post more about this when I figure it out…