A small image used as a preview of the level you are about to enter.
Almost an Omnipresent Trope in Video Games, this trope is about any kind of visual aid that helps players telling levels apart with a small image depicting a part of the one they're about to access. Imagine a postcard for a level. The Level Thumbnail can show up in The Hub, in a level selection screen, and even in a World Map (although more rarely). This being such a common trope, listing examples might even be redundant, but notable ones are still welcome. Level Diorama Preview ( only a not-yet-launched ykttw at the moment) is a Sub-Trope, about a more specific and more detailed variant of this.