I've been searching the forum for help on my topic..and after an hour this seems like the best thread to post to. I used the tips above to determine that my ARCHIVE page is using kubrickbg.jpg. I would expect it to use kubrickbgwide.jpg, because the class in the archives.php file is "widecolumn" and there is no call for a sidebar.
All I've done to the template relative to the image files is replace them with my own, so just to be safe I verified that the "wide" image is all white (no grey bar).
Why is the "if" statement in header.php pulling in the wrong background on the ARCHIVE page? It shouldn't have the grey sidebar. My archive page is here: