I think that the url to the imagesfolder is set in the var $includes but don't ask me how to change it... You could change in the skin files every {$includes} into your url (https://forum.cacert.nl/servers/1/templates/79/includes) but it would take lots of time, and your forum won't work yet, because the pages aren't monitored neither...
You could alsoo give it a try to install the software on a free hosting, and opening it in a frame on your website...
Anyway, if there is a correct and easy way to fix this, Chris will help you