Well, frankly, when starting a game I believe it can be part of the fun to not exactly know where the others are (in most cases you can guess). This is also the reason for the "fog of war" and the need for scouting.
Even more, if you are playing nomads, you may even not be able to determine beforehand where the CCs are but only where the starting position is.
As you can dynamically select for some maps how many players participate, it will be challenging to show the positions on the static preview.
Providing a map preview serves, as @hyperion pointed out, to recognise a map and to get an overview of what this is about topographically (desert, jungle, mountains, sea scape...). Sorry for being so negative, but I personally would probably not overload the preview.
Best regards,
Grautvornix