Relative URLs should be resolved in aggregate stylesheet.

Relative URLs should be resolved in aggregate stylesheet.

E.g.
/sites/all/modules/module1/module1.css

body {
  background-image: url(img/module1-background.jpg);
}

When this stylesheet is aggregated "img/module1-background.jpg" will point to "/sites/default/files/css/img/module1-background.jpg".