Today, Google has enabled the option to display a Bing like  homepage background image on it’s homepage. You can choose an image from a collection of images provided by Google or choose an image or photo from your computer,  or from your own Picasa Web Album.

To change the Google homepage Background Image,  click the homepage link “Change background image” located at the bottom left corner.

I see this feature has not been enabled for some of the locals such as  google.lk, google.ae etc. But you can still access this feature by visiting Google.com.