Problem with creating accurate heightmap

Hi, I am new to mapping and working with tools like world machine in general but I wanted to give it a try anyway.
I want to create my heightmap with a pixel editor and since I do not own photoshop I thought I can use Affinty photo.

My idea was to generate the different terrain heights in the FAF map editor, so I know the exact grey value for each height.
Turns out Affinity can not open .raw files. I then read you could use gimp if you rename .raw to .data, which indeed opens the image but the grey values are wrong for sure.

My next thought was to generate different grey values in Affinity Photo and see how it turns out in the map editor but I can not import a height map as .png like I read somewhere else.

If you have some advice on how to solve this problem, without using photoshop, this would be very awesome.

Best Place to get a Fast Answer is to joining the Discord Assign yourself the Mapping role and ask their that were most people are

"The needs of the many outweigh the needs of the few" - Spock

Thank you! I will ask over there.