Make sure that it is located 2 directories above the location of your style sheet in the images folder (did the orginal image get deleted or over wrote).
You can also add an images directory to your theme folder and copy the image in there and change the line like so:
background:url(images/bg-three-column-main.png) repeat-y;
this method is better when adding multiple themes to the site that may change dynamically according to who logged in.