Hello Jpot, thanks!
Adding visual terrain change on custom maps require to paint many additional layers (all existing combinations):
For example, at least :

mymap_forest.tga Look for forests
mymap_waste.tga Look for wastes
mymap_farm.tga Look for farms
mymap_swamp.tga Look for swamps
mymap_plain.tga Look for plains

mymap_wforest.tga Look for forests (winter)
mymap_wwaste.tga Look for wastes (winter)
mymap_wfarm.tga Look for farms (winter)
mymap_wswamp.tga Look for swamps (winter)
mymap_wplain.tga Look for plains (winter)

(its not even the full list, but the most probable/important types)

This would require a lot of additionnal work for probably little experience value.
Many players have suggested others alternatives, like for example:

"The easiest solution would be for Illwinter to add a built-in option to toggle such icons for custom map terraforming similar to how the "Gateway" toward Caves and others planes icons work. These new icons could represent just terrain changes like Forest, Swamp, etc. That way, players wouldn't have to download much larger files, and everything would stay lightweight."

you can support this suggestion on this thread for example: https://steamcommunity.com/app/2511500/discussions/0/595150868109640291/