You’ll have to play around with the image size a bit if you don’t want to use a repeating image. I had to make mine about 1.5 times as wide as the notification center to get it right because OS X automatically tiles whatever image you use, but I’m an idiot and I got it to work.
I should note that Alexandra Douglass’ “It’s Summer!" image has been my favorite wallpaper since she posted it. She’s fantastic.