Monotone: adding “real” thumbnails to archives

I’m currently using the theme Monotone, and I think it’s very beautiful. However, on the archives page, the thumbnails to each post come up as “fake” thumbnails (i.e. the full-size pic resized to be thumbnails). They don’t look as nice and take much longer to load than if I were to use actual thumbnails. Here are my archives. I wanted it to be like Monotone’s demo archives page, found here. Any idea how to do that?