There is a bug with the TileMap object related to the image size vs tile size (it seems to crash if the tile is bigger than the texture itself), another report here: [url]Crash when I try to use a tilemap object D:]
This should be fixed for the next version (hope it’s your bug) 
You can check if the image mode is RGB (not Indexed) too, you can do it with GIMP (Image >> Mode >> RGB) 