Ok, I have a simple black and white image with a X and a O. When I go to draw them, the X isn't transparent. The transparent part of the image is black, but on the O it is transparent. Color to be transparent is white.
Probably, the color you think is white in our image isn't quite white. If the color values are, say, (254,254,254,254) you could never visually tell the difference, but Irrlicht can