If you headed to Google.com today and found the Bing-like background image enabled by default, don't freak out — it's apparently a 24-hour introduction of the feature, with backgrounds rotating on a random basis.
UPDATE: It looks like enough people were upset with the change that Google decided to remove the auto-background feature.
If Google.com's a regular part of your day, and the background's distracting, do what commenter Cathoo recommends: hit the "Change Background Image" link in the lower-left, head to the "Editor's Picks" section, then scroll to the bottom to find the familiar, entirely white "theme". Or just wait until Google's background experiment reverts to its mostly-blank default. [Official Google Blog]