"I give credit to whoever posted the background onto google"

Yeah, that's not how that works. 

"It came up in Google Image Search" does not equal "this image is in the public domain and I can do whatever I want with it." Shit, guys, it says right here:

And even if whoever actually owns that image is cool with you using it in your work, "whoever posted it" is not proper credit. Drop a name and a link. Maybe shoot a "hey I used your cool thing, thanks" e-mail off to the owner. People who make stock art enjoy seeing what other people are doing with their images.

If all this "credit" business is still too much trouble for you, let me introduce you to morguefile.com (not my site, I don't benefit from that link, it's just a resource I use the shit out of). It's full of actual public domain photos you can use for your backgrounds or whatever. Yes, public domain. You don't have to credit or link back or ask permission (though it's still great if you do the first two just for shiggles). It's great. Use that instead of using someone else's work without permission.