1) Make sure your blog is set to the classic minima template. Help?
2) Find the freebie you want and click the image.
3) Copy (ctrl + c) the url at the top of the page (address bar).
4) Go to Dashboard > Layout > Edit HTML.
5)Look in all the code for this (its near the top) :
Replace it with this:
background: url(http://www.photobucket.com/background.jpg) no-repeat center fixed #FFFFFF
Replace the URL above with the one you copied earlier. Click save, and voila!, there you have it.