Wrong size of post thumbnails on posts (Pinboard Theme)

I’m using the Pinboard Theme and I’m very happy with it so far but I got one problem with the size of the post thumbnails that should be displayed over the hole width of the text.

My screen got a resolution of 1280×800 and the picture only spans 2/3 of the text if my browser is on fullscreen. If I narrow my browser window the picture resizes to span all the text correctly. On may droid-tablet everthing looks fine, too.

See here: http://klassikeraufderstrasse.de

Thanks in advance for any help.