I had my TF2 server set as a L4D2 server on Gametracker. (Because I switched which game I was hosting on which port.) I changed it to a TF2 game type, and the map thumbnail displays correctly now. (Problem solved.)

However, I noticed if you have the server set to the wrong game type, such as setting a TF2 server to L4D2, (like I did by changing the ports around on my server,) it doesn't show the map image, even though it correctly shows the amount of players, who is on the server at the moment, and all other information correctly.

Map images are unique per game. If there is an mp_broadcast map image in COD4, that will not be used for a map called mp_broadcast in TF2.

Many games will continue to scan even if the game is display incorrectly. This is because games, especially but not limited to, in a series use the same query protocol.