Thank you, Lynne!
So i found this code in postbit_attachmentthumbnail, which seems to be the link used, when opening an attached image:
Code:
<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&d=$attachment[dateline]" rel="Lightbox_$post[postid]" id="attachment$attachment[attachmentid]"<if condition="$show['newwindow']"> target="_blank"</if>> thumbnail code removed</a>
So I guess I would have to make changes to attachment.php, in order to not only show the plain image, but a html-site with the image and the desired background color?
I don't know much about php, so can this be done in a few simlpe steps?