Hi Mathias,
Because of the way our CSS Handler combines and minifies the CSS it also has to resolve relative image urls in background images to fully qualified urls. It is very important when defining background images in your CSS to always use single quotes around the image name like this:
url('someimage.png');
not like this: url(someimage.png);
and not like this: url("someimage.png");
If any image urls are incorrectly defined it can break the skin in unexpected ways.
I also recommend keep it simple by keeping the images in the same folder with the css files that use them.
Hope it helps,
Joe