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.
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…