Seems I have a partial answer to image transparency, at least. The release notes for Kindlegen version 2.5 build 0626-3a91e28 includes
Quote:
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Improved transparent PNG conversion: Transparent PNG images (now converted to JPEG) will use white background. This originally used black background during conversion causing degraded reading experience.
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So not only will the transparency not work, it will also trigger a PNG→JPEG conversion. (ETA: Experimentation shows that kindlegen actually converts to a GIF image, without transparency.)
Well, that was worth a try, anyhow; whether EPUB e-readers support transparency (and what happens when a white-on-black theme is applied) hardly matters if the primary audience won’t see it.