Is it common to call these sprites in British English? I would call them "thumbnails".

The term "sprite" is mostly used in games. A single image file that contains multiple sub-images is called a "sprite sheet". (This reduces overhead compared to loading a large number of image files.) In this case they happen to be video thumbnails.

I'll admit I was expecting an explanation of how to extract sprites from retro video game footage using FFmpeg, which I can see totally being possible with a bit of work.