Set Thumbnail
By default, the thumbnail for all maps is /assets/maps/default.png
To set a thumbnail for maps, upload <id>.png to the /assets/maps folder, where <id> is the map id.
As shown below, only maps 1 and 3 will have a custom thumbnail, all other maps will use default.png
The default.png is below, but it can be changed to any png you wish to use.