This url shouldn't work at all. Given my earlier comment, I presume it should be `../img/whatever`. But, given that we have file sessions, this still won't work, i.e. use embedded base64-encoded images. @bryevdv, @pzwang, was this issue every acknowledged? I think we may not want to put all images in CSS files, but in this case we would have to extend `HTMLFileSession` to automatically embed images.