Continents generation

This is my first post on this forum.
I want to create game with some continents which shape I want to make manually. The easy way to create base for continents, and manually build mountains later. The way I want to build continents at sea level is attached. I converted that into raw by some way. I want to use that as a heightmap for creating continents and later adding hills manually. How I can do that if I already know shape of continents?

In a typical situation, you can use a heightmap to create a landscape and then you can sculpt and modify the result as much as you want afterward.