The screenshot pictured above is actually finding threads via a search engine, and so they pull images from threads into their own web search.
We're really sorry, unfortunately, there's not much we can actually do about this. It's out of our control as it's not us doing it, however, we can confirm that we don't use thumbnails on our internal search. The search bar at the top right is external and does search the whole internet rather than just Mumsnet!
Basically, nothing to worry about but not something we can change at the moment.