Javascript is case sensitive so have a look in the img/tilesets directory in the www directory and see if the file has been named wrongly, e.g., fullmastback-B.png. If it has rename it as FullMastBack-B.png and see if that fixes it. If you are silly enough to play the compressed version of ToN this kind of thing is want to happen. (Usually game files are named using Camel notation.) Other than that the file will be missing or corrupted.

Maybe somebody will post a copy of it for you to download if you ask nicely. If all else fails download the full uncompressed game and enjoy.

Nil desperandum.

Worse things happen at sea.