[DustyPorViva]

Ok, I'll answer it here...
addtiledef2 is very similar to addtiledef in functionality, except that it's overlays an image over the tile, instead of replacing it.

The prefixing works the same. If you do:
addtiledef2(imagecrap,"ow_",0,0);
It will replace the tiles at the 0,0 pixel coordinate in the tileset for all levels starting with ow_.

Now, you can also do:
addtiledef2(imagecrap,"ow_a01",0,0);
to replace tiles specifically for level ow_a01 an none others. I suggest this whenever you use them on the overworld, so you don't get unintended tile errors.