Google Hosted Website Background Images?

I stumbled upon this URL:

http://themes.googleusercontent.com/image?id=1uK3asOUTXGHOxBp5SzQi9-XgNXEKZ36dz3ulQ9zIinkHa_wkts_jwC2cKfeghsEwd8xV

and

http://themes.googleusercontent.com/image?id=1OACCYOE0-eoTRTfsBuX1NMN9nz599ufI1Jh0CggPFA_sK80AGkIr8pLtYRpNUKPmwtEa

Where are these images from? Where can I find more Google hosted background images to use in my web projects? (Using a Google hosted background image can save on bandwidth and speed up a website.. every bit helps..)