That pic is straight out of one of my AvA terrains.
Here's the rules for what you can/cannot change in an MA terrain:
1)
Editing terrain texturesYes
2)
Rearranging the object layout of the default terrain clutter tilesYes
3)
Replacing terrain objects (like trees or forrests) with custom objects on the clutter tilesNo
4)
Editing default object texturesDepends. One of the goals of a MA terrain is to keep the size reasonably small. Changing a lot of textures have the greatest imapct on terrain file size.
5)
Rearranging the object layout of the bases or stratsNo
6)
Replacing base or strat objects with custom objectsNo
7)
Adding new objects (like bridges)No
So, according to Rule #1, yes, a winter or desert terrain is allowed.
