Google is rolling out a new features (Inspired from Bing ?) which will enable you to set a background for the search page. Google says they are rolling out this feature for the users from the US only while others will have to wait to see this feature.
If you are from US you may (may take more time to see this for all UC users, says google) see a link lower left-hand corner of Google.com. You can use an image from your computer or Picasa.
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