[DustyPorViva]

I suppose you want more colors? Ah... new technology.
Anyways, first, to make sure it's not your script and it's your image. Just pop an NPC in the level and set the image of the NPC as the tileset. If the image is correctly saved and will work in editor, it will load. If it loads, but doesn't add the tileset, it may be your script.
To clear it up, first delete tiledefsoffline.txt in your tiledefs folder, and try the script again.

As for more colors in an indexed png. You can't... but. You can add the tileset template as the tileset, and divide the tilesets into different parts, and add them via addtiledef2.