This is a great little map editor - the only reason I don't actively use it however is because it desperately needs the ability to export to png and json. So far the options are only "map", "level" and "retro map data".

Thanks for the feedback. I'll look into adding that functionality. You could write a little program to convert the map file to json. 
iirc I tried to explain how the map data works in the readme.txt. I believe the "retro map data" is a raw data dump separated by spaces so you can see it in a text editor and import it into anything you want or convert it. If i get around to it, i'll try to make a program to convert the file and upload it for you.

I haven't worked on it in a while, so i'm a bit rusty. I actually have a newer version or modification of it that i've been meaning to upload which edits chunks of a map and stores them in one file for a roguelike i'm working on. each chunk is a room like binding of isaac or a screen in the legend of zelda. If that would be of use to you, i could upload that too. 

edit: Oops! I just checked and i guess I didn't specify how the save data works in the readme.txt. 

I thought it would only take an hour, but I ended up spending a few hours adding that functionality for you.  It will now save and load json maps and export a png of the map. The Fileinout extension broke, so i removed it for now in this version, which means it will only save in game makers allowed sandbox areas unless you're running the project in the editor. I added the project to the download page and tried to update the game example project Save and Load scripts which should work, but i didn't have time to test. 

Thanks! You did a great job.