As far as I understand the problem, the upload can't be 1 to 1. The reason is that the Widelands map is triangle shaped, and bitmaps are square shapes. Of course, there is a "simple" function that can translate one dimension to other one, but it will change the shape a bit. Also you can develop another function that will preserve shape, but will not preserve areas.
But if you don't care about details (really, details!), you can ask Tibor for a script. He generated some maps with a script, but I am not sure what the script was doing inside. As far as I remember, it was generating lands and oceans by some initial conditions and translated result to Widelands map. Your idea is different in only one point: you don't generate map, but open a file with existing idea.
But to be honest, if you search for maps in "maps" section, you will see many maps with high details. It takes some hours to make one, but it is possible. And some of the maps are designed tail-by-tail. Examples:
And some maps are well-detailed, but not done tail-by-tail (my two maps here):