03-02-2021, 11:18 AM
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