Say, you have a CSS file in `webroot/css/something/main.css`. In there is a `url('images/sprite.gif')`. So the path to the image would be `webroot/css/something/images/sprite.gif` as CSS urls are always relative to the CSS file (unless they start with a `/`). 

So with your code the file would be searched in `webroot/images/sprite.gif`, right?