Probably because the underlying GD Library doesn't have support for Windows .BMPs.

And that's probably because there is no existing open C library available for load/saving .BMPs as there are for .jpgs, .gifs etc.

And that maybe, though I haven't checked, because MS have never freely published the spcs for .BMPs?

Although my snippet works, for simple, uncompressed BMPs, the references I've seen suggest that 'the spec' has a multitude of extensions.