>> How can I make a custom minimap in the game menu screens?
Very simple. All you have to do is open the map (4)Monolith located in warcraft III\maps\frozen throne\scenario\ and go to the import manager. Export the file called 'war3mapPreview'. Now use photoshop or whatever to edit it to look like whatever you want, then save it with the same name and format (.tga).
Go into your map and import this file. Double click it and edit the file path so it only says 'war3mapPreview'. It should now automatically show up.