Hello. You are welcome to use any of the backgrounds in this directory. You cannot directly link to them from your page (or at least, I think I have it set up so you can't). Read the FAQ (link below) for why this is a problem.
Besides these backgrounds, you are also welcome to use any of the backgrounds on this website.
To save a background, right-click and choose Save As, to save the file to your own directory. They are all bigger than the thumbnail. To see the actual size, right click and choose View Image --> foreground tab.