Very cool, great execution for a wallpaper site. Few suggestions...

1. Filter by color. Allow users to pick a color on a palette, and select all the images with that primary color.

2. Categories. A simple dropdown in the navigation to select nature, architecture, water, abstract, etc.

Lastly, why don't you branch this concept out into other areas? This would be a beautiful showcase for general photography. Allow users to upload their work, 'like' photos, and display the best on the homepage. Mix in profiles, and the ability to comment on images you select, and it would be an interesting site. Somewhat of a behance, pinterest, logopond mix geared towards photographers.