SSL Failure because of mixed content

Hello, I've ordered an certificate to our domain port-au-print.de to access the site with SSL. Now my browser (firefox) always says that there are mixed (http and https) contents, so the site isn't trusted. I can't find the picture that causes that fault. Is it possible that pictures like favicon or header are always only accessable with http? I need a solution for this. Can anybody help me? Even in the backend I got that certificate error. He marks 3 code-lines...