I can't locate the html that puts the image on the page.
Have you got the html open in some type of editor? Search for the string <img If it's an ordinary image displayed in the ordinary way-- not yet a background, the way you want it to be-- it has to turn up. Probably very near the beginning of the file. That is, ahem, shortly after the word <body>. I don't know how much clutter is in the <head> so far.
It would be easier to answer if we all had a clearer idea what your current knowledge level is. If you're helplessly staring at a wad of HTML feeling like a monkey looking at the innards of a Swiss watch, that would need one type of answer. If you're accustomed to HTML and CSS but have never happened to do a background image before, that's a different type of answer.