It's a simple enough feature that we've heard rumors of here and there: Just go to Google.com,Pick any image from Google's Public gallery, your Picasa web album photos, or upload any image from your desktop
For this do following steps ..
1. open your browser and go to picasaweb.google.com
2. Log in to picasa with your Google ID and Password
3.You can select any albums and select upload in the top
4.A new window opens with Drag your Photo here ..or with select photo from your computer
select the photo that you want to make the homepage image
5.Click OK ..
6.Now go to www.google.com
7.Select the link Change background Image in the bottom left corner
8.A new window will appear to select image for your google home page. Select Your Picasa Web Photos or Your Recent Picks from the side tab on the window
9.Select the Picture that you want to make the google home page image ..
The google page will load your image as home page .. and njoy...
you can also set the popular images in the picasa web albums...