Thanks so much to everyone for the help on this over the last few days.
I figured I'd post it here so you can see what you've all be helping me with. :-)
Godot 2.1.3 on a Mac.
Saving out the data in JSON format and reading it back in to re-draw the editor tiles.
Each level saves in its own text file. For example, 1.txt, 2.txt etc.
The images/bricks in the dropdown are auto loaded out of the texture/bricks folder when the app loads so I can simply drop PNG files into that folder and they populate the dropdown. Very cool.