Doing this in CSS is almost imposible.
You could also set that css line from the index.php and use the PAGE_ID variable to create a filename.
Just make sure you have an image for all pages (or build some php that checks for file_exists).
I could think of more tricks like this.
i.e.: Use the keywords field to set the name from the backend (and don't use the dynamic keyword META tag).
or: select a different CSS on the PAGE_ID of the current page.
There are several examples on this forum.
Have a look at this:
http://www.websitebaker2.org/forum/index.php/topic,9284.msg54875.html#msg54875Ruud