Let's Play: Dungeons and Dragons Online. Stromvauld's mine
So, youtube offers 3 different scenes from the video to make into the thumbnail. I can however make my own thumbnail, via paint or photoshop or something. I can also choose a different scene besides the 3 they show.

99% of the time i just pick one of the 3, the 1% of the time i don't, are videos where something really noteworthy happens. I level, I die, I get great loot, things like that. The other reason would be that all 3 that youtube gives suck.

50% of the time i just leave it on the default one.

this was one of those times.