1. Go to Blogger Dashboard.
2. Click on your Blog Title.
3. Now navigate to Template tab.
4. Click on Customize button beneath your live blog preview.
5. Now you can change background image or color as below.
Change Background Image
i. Click on Background on left pane.
ii. There you can find heading Background image and click on the image box below the heading.
iii. Either you can select background image provided by Blogger or use Upload image menu item to upload you own image.
iv. Click Done button after the completion of upload.
* If you wish to customize font and colors of your template, read How to Customize Fonts and Colors in Your Blogger Template.