There already is one. Made by someone called Retalyx
You just load a picture, black is used to place stars, other colors are blank areas. I sometimes use other colors to help mark out pictures because the program will just view them as blank space.
It's currently abandoned. Which is a shame as it's very well done. I recently made several maps for it only to find out that it doesn't generate fallen empires (I think it did with an earlier version of Stellaris though). I don't know how it handles all of the new features that have come out since it was abandoned (and new patches could break old features).
I don't know if the creator would be ok with someone taking it over, but hopefully someone will.
In the folder whichever file is named galaxy.png will be what gets used to generate a galaxy. You can also randomly generate a map.
Hyperlanes can be randomly generated with options for how many lanes you want per star and how long they can be. You can manually add hyperlanes (I can't figure out how to subtract lanes) you can manually add move or delete stars. You can add nebulae as well. Using the maps disables achievements (they load as mods).
http://steamcommunity.com/sharedfiles/filedetails/?id=699098812
https://steamcommunity.com/app/281990/discussions/0/357286663685698126/?ctp=2