I have a CSS file with my theme and it references background images as background: transparent url(images/Banner.jpg) no-repeat;

On my install some work correctly reference and display the image OK and others fail and show the path:
index.php/tools/css/themes/minifiedtheme/images/background.jpg

The only way around it seems to be making the url absolute within the CSS which I don't really want to do.

Any clues as to what might be causing this?

Thanks in advance