Height map is type of Objects Map (Used for W4). This type used for creation of landscape of map.
For import from image Height map use Properties Panel - Height Map - Import Hmap. Will opened Import Height Map form.
You can also Export your Height Map for edit. For export your Height map to bmp-image use Properties Panel - Height Map - Export Hmap.
Landscape W4 have also Mask for Second Texture. You can Import/Export your Mask for Texture Height Map. For import your Mask for Texture from bmp/jpg image use Properties Panel - Height Map Texture - Import Image. For export your Height map to bmp-image use Properties Panel - Height Map Texture - Export Image.
All commands analogic to Import Height Map Form.
In Worms 4 Mayhem you can also change Mask textures of Height Map. You can find Name of Base/Second Texture from Worms 4 Mayhem\data\databanks\mapname.xml file.