Problem solved, but I'll leave it here with the solution in case anyone else has the same issue.
I'm having a strange, and very frustrating problem.
Is passes the W3C validator, but I can't get the background image to show! The headerbg.jpg file is a valid path, and is shown on the page using img tags. When trying to set it as a background, it doesn't seem to work. What am I doing wrong?
Here's the css.
background-position: 50% 100%;
padding: 0 15px;
Found the problem, was a silly one. I thought the css file loaded from the original source files directory. Nope, loads from the css file.