Firstly I tried a simple image map node and got a strange background of black and grey striped triangles instead of transparency.
Then I tried a default shader same result but all other landscape colouring turned off. Main landscape colouring plugged into input.
Then I used two separate image maps one for colour and one B/W image for transparency - same results.
Finally plugged B/W image into blendshader of image map and then into a surface shader child input this worked.
In all case used a separate camera to position image map
I now have another problem the image is far too bright I can sort of control it with the gamma settings but it is not ideal.
And can we please have greyscale/partial/graduated transparency, please, please, please