1. Login to Blogger

2. Go to Design and select Edit HTML tab

3. Now find [CTRL+F] this code:

body {

The BODY code will look like the following one

body {
background:$bgcolor;
margin:0;
color:$textcolor;
font:x-small Georgia Serif;
font-size/* */:/**/small;
font-size: /**/small;
text-align: center;
}

4. Now Replace the above Blue Color (background) line with the below line

background: url('http://DIRECT LINK OF YOUR IMAGE') repeat;

Replace the Red Color line with the direct link of your image

5. Now save the template and a background image has been added to your blog.