Thumbnail problem

It looks http://jakobgravlund.dk/ is using http://demo.woothemes.com/aperture/ by WooThemes.

Example link after clicking on the first video post: http://jakobgravlund.dk/?p=345

I’m not familiar with the Aperture theme itself, but it’s possible there’s a setting for how featured images work for it in Appearance > Theme Options. Just to make sure, have you checked all of the options there?

If there’s no option, you can probably adjust the theme using a child theme (which is a good route for making modifications because it lets you keep your updates separate from the main theme files). You would need to figure out which theme file controls the pages you want to modify, duplicate it in a child theme, and modify it there.
http://codex.wordpress.org/Child_Themes

You might try http://www.woothemes.com/support/ for help too, since they are the theme author and have a support site.