Yeah, if you could save like the physical map of the world in an image file to look like a map, it would be very good for worldbuilding.
Isn't that feature called Screenshot ?
Use the ingame screenshot or press "Print" to copy it to the clipboard and insert it your your favoured image view/edit tool.
He probably means steganography, which is saving hidden data in an image. For example, Terratech game uses it to save the vehicles. You take an ingame screenshot of the vehicle and you can load that same vehicle from that screenshot in another game.
I dont know how it would improve rimworld tbh. Right now is very convenient to have a flat txt file that you can edit manually, and if what you want is to save the world map itself and nothing else all you have to do is just remember the seed.
My interpretation is that he wants to flatten the roughly spherical world map comprised of hexagons and 12 pentagons into something resembling a Mercator projection.
Worldbuilding makes me think he is going to engage in the ancient art of tabletop RPG.
In which case it would make sense to use an existing map generator, such as https://www.d20srd.org/fantasy/world/ (https://www.d20srd.org/fantasy/world/).