GnuTux made it so that very large images that would stretch out the thread would automatically resize so that the image and the thread page wouldn't have to be scrolled from side to side to view it. Once a too large image is in place, the whole page of the thread results in trying to view the image and read text by scrolling from side to side without that feature in place. That's something I find very annoying.
Clicking on the image enlarges it and also it can be sized to full screen. It's not a thumbnail and it is large enough to see some of the detail in the image.
I'm not sure far down they go but I believe it depends on your screen resolution.
I personally love the feature. I agree, it is somewhat of a surprise when a large image comes up then resizes.

GnuTux will have to explain the technical stuff behind it.