Woah, this is ridiculously underrated! it took me a few minutes to figure it out, but once you do, this is an incredibly useful tool to jumpstart tileset creation! I'm actually working on the level chunks for a little fangame called Sonic CD 2, and this has helped a ton! :D
Here's the tileset btw :P
Viewing post in Sidescroller Tile Set Maker comments
the only thing I'd add is an extra version of the first two tiles on the bottom row, where it doesn't use the "bottom" tiles, so that tilesets can still have a bottom tile without having tall walls have the bottom tile keep popping up. but other than that, it's a great tool, thank you so much for making this! :D