Digital Blasphemy (named for the "godlike" sensation of creating new worlds) has been providing high-quality original 3D artwork for use as computer backgrounds since 1997. This app makes it easier than ever to browse the hundreds of different wallpapers in the gallery and personalize your background and lock-screen with one touch.
The App itself is FREE but a paid Membership supporting the artist is required to download most of the backgrounds (registration link is found within the app). There is a collection of free wallpapers to get you started though.
Your Membership login is stored in the app once entered so you should never have to "log in" more than once. Members can leave comments and ratings on the images to help the improve the gallery.
Once you've found the image you want, one-touch will set both your background and lock screen. You can also download other resolutions should you prefer.