Hi Nicklas,
Super sorry about that – this is a known bug that I overlooked when I released the new version of the Blog addon – I guess I was too excited to show off the new features.
The fixed themes have been uploaded to WP.org, but the queues over there are pretty brutal, so I made a post below that gives you the updated functions.php file that you can replace.
http://generatepress.com/forums/topic/double-image/#post-47009
Simply copy the code from the “Exhibit” file I posted, and replace everything inside “Appearance > Editor > Functions.php”.
Let me know if you have any questions at all – and sorry again for the inconvenience.