The first thing to do is to download the GIMP plug-ins. There are three layers that are used for elevation, terrain, and trees. White is high, and black is low. You can choose from three terrain types. Those surfaces affect his traction and how he slides.
Trees are colored dots on a black background.
Black pixels simply mean no tree, while any other color is a tree. One pixel is one tree on the map. Editing the elevation, terrain, and tree layers will require you to flick among them individually. That will provide a pop-up you can use to look at the layers and make changes where needed. The name of each layer also appears on the pop-up.