I had the same experience. It changes back to JPG every time I click the File > Edit option. I did find, however, that if I edit the .JSON file with a text editor, change the value of MapImagesExtension to "png", then load the .JSON back into the editor, as long as I didn't click the File > Edit menu, I could export the images via Export > All and they were created as PNG's and worked great on Nitrado. Note that you now have two images being generated for each cell (ex. CellImg_0-0.png & CelLTradewindImg_0-0.png) and the MapImg.png.