Patches, objects, monsters, and files for H3WoG.

I don't understand something, or is there an editor that creates terrain based on an image?

There isn't such an editor, but there is a patch for it - H3WMapEdPatch.
The patch provides support for new monsters, artifacts, etc., and adds additional features:
Additional menu items, Event Linking, inserting commanders and emissaries on the map and in armies, WoG button panel, fixes for random map generation.
In addition to the patch, there is also, as I already mentioned, the H3MapTool program, which specifically deals with inserting images!