Glen,
1) You asked: Aside from the login difference, are you going to want to account for any pages that you cannot do with separate css files?
Answer: First, I thought that I could only override the headerbg.jpg image in the separate css files. (I believe that I would also need to override the searchbox.gif image)

2) You asked: is there any of those images that needs to be a foreground image, or could they all be backgrounds?
Answer: Since I am a novice I do not know the difference. If you could tell me the difference I could then answer.

I assume you asked the foreground/background question because of your two followup statements.

A. The code in post 151 will handle one foreground image filename, though it would be simple to apply it to several images that switch in the same way. It does need to be used as a unit exactly as I posted it, replacing the few lines of your original code as noted. With a little modification, it could be used to switch both gif and jpeg files.

B. "... tweaked to give unique body class names in tpl_main_page.php, and handle all the image switching as backgrounds without any additional stylesheets (all the alternate bg names would go in the main stylesheet)."

So, I guess my response foreground or background will mostly depend on what the differences are between the two.

So, why use one over the other?

Sawhorse