Thumbnails have low resolution (GPP Fullscreen)

Hey.

I’ve recently installed the Fullscreen theme by GPP and I’m having an issue. All the images display just fine, however, the bottom row has extremely low resolutioned thumbnails. Any idea as to why this is?

You can see my frustration at https://riotography.net

EDIT ADD: Seems like someone emailed me rather quickly to change thumbnailsize in the mediapane as well as install the regenerate thumbnails tool. Thanks for the quick reply.