Super imposing two images

[tmanni]

Another way without using layer mask nor selection :

• create a layer group and move the map layer inside it
  
• apply Colors > Color to Alpha... on the map to send the white color to alpha channel (decrease the Opacity threshold to keep the dark area full opaque)
  
• File > Open as Layers to import your second image and move it inside the group, on top of the map layer
  
• double-click on the second layer to open the Layer Attributes dialog
  
• change the Composite mode from Auto to Clip to backdrop
  
• finally, create a new white layer for the background at the bottom of the layer stack