(This is not what I am trying to do, but it is a simple explanation of my problem...)

Suppose I have drawn a round ball in MSPaint, with the ball white and the surrounding image black. How can I texture map it to a quad without the black coming up (ie, the surrounding area is see-through, as if the quad was in fact circular)? OpenGL doesn't seem to do it automatically, and the red book doesn't explain how to do it.
Do I have to store alpha-values in the picture? Is there a special blend function?

Thanks,

me.