Plus, to stress it again: the parts you want to be seen have to be completely white and the rest black. A simple grayscale image in the middle window will make everything transparent in varying degrees. (Good for other effects, though.) Also, the bits you want to be seen have to be completely inside the photo. No intersecting with the edge of the photo, cause you won't be able to get a proper result in Bryce. And yeah, as you can see in Zhann's pictures, transparency value has to be zero for "blend transparency" to work properly.

-- erlik