Oxygen theme: images not showing when viewing posts, filtered on category

Hi,

Got a weird problem. The oxygen theme works like a charm, except when you click my category menu, or try to view posts filtered on tag. The images featured in the posts are shown in the overview as a grey block. They should be exactly like the front page… Can’t figure out what’s wrong. Anybody got an idea? (www.checkhet.net)

Thanks in advance!