Images in CSS files not loading when using $this->EE->cp->load_package_css()

In my CSS files I’m using some background images which are all in my third_party/[add-on]/css directory. But the images aren’t loading when using $this->EE->cp->load_package_css().

If I load the CSS directly (by including a <link …) the images appear.

So when using $this->EE->cp->load_package_css() do the images need to be in a certain directory? Or called in a certain way?