I’m trying to remember the name of a UI control that could be backed by something like this. It was a sequence of thumbnails laid out in a row like a film strip. (For all I know, that was what it was called.)

Could be a poster frame or chapter frame or chapter marker

Yes, and the sprites were supported by a specific coprocessor. For example, the Apple II didn't have one.

We had to make do with Shape Tables in AppleSoft BASIC…good times drawing those out manually on graph paper. And then XOR-ing them onto the background. Amazing how good games were on the Apple ][ without dedicated graphics hardware.