In Photoshop, making a heightmap is time-consuming but not difficult.
Pure white (aka. RGB: 255, 255, 255) represents the maximum height (which in this case is 1024 studs).
Pure black (aka. RGB: 0, 0, 0) represents the minimum height.
So if you want to make a mountain, draw it from a top-perspective starting black going up to white gradually.
Example:
If I’m correct, this image should create a very rounded hill in the centre.
Edit: I now made a clear tutorial in case you need more help: Guide & tips on making a heightmap & colourmap for generation