Alpha Channel

I am trying to alpha out the parts of a polygon my image is attatched to that is not covered up by my image. I made a person inside a globe type image in PSP.The image is 32 bit. The trouble is that when I hit front projection under the color channel to map the image to the polygon, I lose the "bubble," and the image jumps forward. Does any one know how I can map this image onto a poly and key out the part of the poly not covered by the image, retaining the round shape and bubble on the image?

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