Map of biomes like in LOTR mod

I decided to check how biomes are generated in mod about the lord of the rings. In the mod file there is a "middle_earth.png", presented as a map of Middle-earth, and each color on it determines which biome will be generated. I am engaged in my fantasy world and would like to find out how to make the same map system for my mod. Please explain how this is done, even if it is impossible to implement it using standard MCreate tools, I will try to understand the code.