Bizarre background-image bug in theme

Did I mention this is bizarre?

Okay, I have a couple of variant-themes. It would probably be easier to use one theme and but various backgrounds and other tweaks in dynamically, but 1. I don’t know how to do it and 2. I think it will load faster if I hard-code.

So I’ve successfully copied a base theme several times and made numerous stylistic tweaks each time. Then I come to a particular domain where I am using a background image called bak-1.jpg. Usually I leave the code here the same and upload a new image, but sometimes I point to another image.

What I first thought that I determined through trial and error is that I get a bug when I use any name other than bak-1.jpg in my style sheet for the page background. The error is shows this at the top of the page:
_gaq.push(['_trackPageview']); _gaq.push(['_trackPageLoadTime']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'https://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })();

God only knows how or why. I monkeyed with it for a long while and then just said, okay when then I don’t have this problem on any of the other domains. I’ll just make a new bak-1 and name it bak-3 for those domains that use bak-1 and then I’ll overwrite bak-1 with the look I want.

But here’s where it takes a turn for the truly bizarre. If I leave the file name the same but change the image to another look—I get the error. In other words, something as simple as a color change in the image makes the page break. I’m at a loss as I can’t even imagine how or why this is the case.