Some themes already include support for adding a custom background image or background color.
If your theme doesn’t include this page, you can add it simply by adding this code in your child themes functions.php file.
The code assumes you have uploaded a default image named bg.png to your child themes images folder.
Here’s the basic code without any parameters:
I added this in my Magazine pro theme functions.php
and background option came but now when I upload image via this option ,it does not stretch ,I am using 960px x 760px image ,I don’t want to do this via this option as it is not changeable through background under appearance ,any help would be appreciated