Oh, wow... I'm so stupid.. I thought you meant my forum profile background, lol. Just ignore the previous message...
I found my desktop background about a year or so ago on Lifehacker's Wallpaper Wednesdays. The exact post where I found it is here:
http://lifehacker.com/5985480/the-nebula-desktop
Wallpaper is downloaded from the artist on his DeviantArt here:
http://tylercreatesworlds.deviantart.com/art/Embers-in-the-Dust-297953167
The wallpaper is really stunning! It is the most perfect work of art for a desktop I've ever seen.
Treat it well! On Windows you might want to re-save the image as a .png in your exact resolution in order to prevent compression artifacts which are really a shame on this beautiful work!