The thumbnail must be 360x180 (or 720x360) in resolution. You can find the property to select the thumbnail image among the map properties, next to the map name.
We have captured all thumbnail images directly through the map editor for our official maps. This is the process we used:
1: First we changed the test options to disable spawning (F6):
► Show Spoiler
3: While in the map enable the debug camera and thumbnail view finder (F7).
► Show Spoiler
► Show Spoiler
6: To open the folder for your newly captured thumbnail press Ctrl+PrintScreen.
7: Go back to edit your map and edit the thumbnail property for your map and select your new thumbnail. Done!