I'm going round and round in circles. I created a couple of templates to test (just simple changes) and then I downloaded a couple of the contributed templates. One of these templates didn't show up the header_bg file - just a plain colour. So I put a question on the help section for contributed templates (no reply so far). Then I started a new template and discovered that I cannot get a background to show up in it either (even if I make no changes at all).
So I figured I must have changed something inside the wrong file. I searched through all the files for changes made in the last month and none of them are located outside of <mytemplate> folder. The original templates I was messing with - Classic contemporary green and my 2 tests show up the background image. The last downloaded template and any new tests I create do not.

What have I done wrong here? Where is the file I messed up? Or how can I recover?