Download Multi Theft Auto/SA:MP Map Construction, there's a map editor in there. You can probably find guides allover the internet to map.
Relevant links:
viewtopic.php?f=35&t=51
viewtopic.php?f=12&t=11837
viewtopic.php?f=12&t=18867
However, the server already got a mapper to do stuff for us, so it's very unlikely that you can have effect on visuals that aren't your house/business/HQ.