How to change Background Image?
1.  Login to your Blogger account,
2. Click on your blog title
3. Navigate to Template

4. Click on Customize button

5. Click on Background on left panel

6. There you can find heading Background image and click on the image box below the heading.

7. Select your new background image

8. Click Done button after you choose background image

9. click Apply to Blog button to commit your changes

Changing your background image is done.