You could turn your stylesheet into a php file so instead of style.css it would be style.php. Then inside your style.php at the top put:
Name your background images and save them in your images folder or where ever you have them like so:
Now in your styles where you set the background:
This should do it but you're still going to have to edit all 300 pages to change style.css to style.php.. I can think of a lot of ways to do it but none that allow you to do it without editing the pages somehow.