Woah, the biggest mistake is to use paintool SAI, because the thing is that you cannot put it as a greyscale canvas. I recommend you to use Gimp only. Try doing it on Gimp again and tell me how it goes. Here are some tips to help you:
1- The canvas size should be 513x513
2- Grayscale canvas
3- Use the colors going from Black to white. Make sure it doesn't have a dot of yellow, red, etc.
4- Save it as .png
5- Make sure the Alpha Channel is not on. Go to Layers > Transparency and if it says 'Remove Alpha Channel' is available, remove it.
Also, make a new map in map maker. It's better to start from scratch than borrowing Watermills. Then, you can save your height map (as .png) in the new folder that will show up. It should be the name of your map.
