Whenever you open google you just see a simple background. If you want to add an image on google as background then it will be so easy process.
For his purpose you must have an gmail account and add your your pictures on google + account or facebook.  Now follow my steps

Step 1
Open google.com.
click on the link lower side of the page '' Change background image''

Step 2
Now you can select image from the google gallery but if you want to see your own image as background then just click on ''your picasa web photos''.
You will see a list of images that you have uploaded on google + account or facebook..

Step 3
Now simply click on any image that you want to set as background image. And click on select button.
Your image must be at least 800 x 600 pixels in size.

Step 4
Now open google.com..
You will see your image as background.. 
Enjoy