Hi Jykke, Wilbur is a height map editor is used quite widely. Terragen is a pretty amazing height map to 3D world generator which could maps just like that from above or any other direction too. Both are free or free for non commercial use.

L3DT standard edition is free and does nice terrain modeling and rendering which includes a 3D viewer and a lot of processing for erosion and so on.

I would suggest trying some of these tools as using a height map instead of just textures makes life a lot easier.

Welcome, theres several good terrain guys here to answer questions or get some inspiration from your stuff.