How to create a geojson file from an image overlay?

I am looking for a way to create a geojson file from the distributions of the health sectors in my region which are different than any political division readily available on osm:

There are fantastic tools such as geojson.io to draw custom polygons but because of the complexity I would need a tool that allows me to overlay an image to use it as a template or that does this automatically. If this makes a difference, I may be able to extract the silhouette with any computer vision library but my problem of how to get from an image or shape to geojson still remains.

Which is the fastest way to approach this?

You can do this using QGIS with manual georeferencing in order to assign real-world coordinates to an image without geo-information. This is done by clicking points on a map that correspond to points on your image. Then once georeferenced you can export the file as a geojson.

Another solution is to find a shape file (maybe this is correct) and simply convert the .shp into a geojson.