Trouble with transparent images

I'm working on an isometric game engine, based on the Dink Smallwood engine. I'm using DirectX 7 to do the usual stuff, but I'm having trouble with transparencies.

I want to have white as a transparent colour for a sprite, and I have set its transparent colour correctly, but it won't work at all. However, black works, and all other sprites from other sources display correctly. :confused:

My theory is that the bitmap uses a different palette than others, and the engine can't work out which is white. Is there any way to get the engine to find out?

Thanks,
DesPlesda