This profanity-laden howto doc doesn't really answer your question, but it does provide a workaround.

FWIW, The IE I have access to shows image 1 at the bottom without alpha, and images 2,3, and 4 correctly. My Mozilla 1.4 shows images 1,3, and 4 correctly, but image 2 is blank, though it did leave a space for it.
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Bitt Faulk