The system is looking for the file in the images directory of your template while you have it in a sub-folder of your main images folder.

Move the file to your images folder in your custom template, then change the image call at line 520 of your stylesheet from
url("images/backgrounds/socialBG.jpg") repeat scroll 0 0 transparent
to
url("../images/socialBG.jpg") repeat scroll 0 0 transparent