Plugin thumbnails in 4.0 – Where do they come from?

I’m just looking for some information. I notice that in 4.0 there are thumbnails for all plugins and that many are obviously added (or auto-generated?) on the fly. I’m interested in doing something like this for post thumbnails. There are currently plugins that allow for setting a default thumbnail, but none that with a more dynamic approach. Does anyone know where the plugin thumbnails come from and if that might be a source for images or at least for a method to do something similar for posts?

Cheers,