I’ve messaged the guy managing the builds on IRC to include the gfx/ folder in the SDL builds. For now, though, if you download the source and copy-paste the gfx/ folder into the game directory it should work as intended.
Or, if you make a new JSON and tileset PNG you can change the path being looked at from the defaults in data/gfx.txt over to the path of the new JSON and tileset image. So, say you made a new JSON called “tiledefinitions.json” and tileset called “newtileset.png” and put them into the same directory as the game’s EXE you’d change the values in data/gfx.txt to “JSON: tiledefinitions.json” and “TILESET: newtileset.png” That will tell the loader to point at those files instead of the default location in the gfx directory.
Sorry for the inconvenience, it should be fixed later tonight.