Assuming V1.5.4, using "Snow" theme.
Say you have a background image named 'background.png'
Put that image in Q2A_root_folder/qa-theme/Snow/images/
Open in some text editor the "qa-styles.css" file located in Q2A_root_folder/qa-theme/Snow/
line 78, find
replace "background:#efefef;" with "background: url("images/background.png") repeat scroll top left ;"
I am assuming here that this image is repeatable in both direction.
For more about the proporties that you can apply to this background (image):