I sketched up two generations of these thumbnails. I'll be showing you the latest as it is the one following the styling of the other mock ups I've uploaded recently and its dimensions are properly set.
However, I'll make the first generation available for view, keep in mind it was literally the first mock ups I made for plasma and using Figma so some things might look a bit weird but I also spent a lot of time on those so it's more complete too so let me know which aspects you'd like to see into the newer generation.

The element displayed are what matters most, the examples I used are for simplicity sake but the behavior is, in my mind, universal across applications, so any program outputting media will show identically set thumbnails (be it Firefox or Falkon or Elisa or Lollypop), any image/picture/window content will be the same too (be it Firefox start page or an image only tab or Gwenview displaying local images).

First generation:

Second generation:

The second of each example is an alternative style which I'm not entirely sure is possible to code.