D3DX supports a wide range of file types for textures. Instead of rewriting everything from scratch, the best would certainly be to use some image processing library to handle them. As a bonus, this would allow lighter code.

Some of those library are :
SDL_Image : easy to use, but loading an image using a SDL_Surface is quite heavy. Moreover, the files supported are not enough for d3dx.
MagickWand/MagickCore, the libraries on which imagemagick is based : Hard to use, and very heavy. Supports every format thet d3dx supports.
FreeImage : Quite easy to use, the only files not supported are DIB and PFM... Not so widely represented. I had to recompile it, as packages for it are not available for 32 bits. (I use a 64 bits ubuntu system).

Here is an implementation of D3DXGetImageInfoFromFileInMemory using FreeImage.

Suggestions and comments are welcomed.

PS : Please CC me your potential answers, as I'm not a subscriber of this list.