Do you want to MAKE the map from the mmap image? - It's very hard.
If you have seen a mmap but you don't know which map ID is, you have two ways:
a) Test map by map until you find the map you are looking for (visiting it with a GM or with a map viewer)
b) Pick a MapInfo.txt and search it by it's name, then you will have it's map ID.
To add the map, you have to add it to MapInfo.txt. If you have found it in this file, then you already had it. Else, add a line like this:
[MapID MapName 0] HORSE
IE:
[0 BichonWall 0] HORSE