Images from one of Princess Tutu's old official websites1, spliced into complete images and saved as PNGs.2
These are some of the highest quality official digital renders I've been able to find.
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You can visit a wayback archive of the site here.
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On the actual website, the menu buttons were made up of multiple non-transparent, non-animated GIF files, each with its own 256 color indexing. PNGs weren't really a thing back then, so giving your website a solid background color, and then using JPEG or GIF files that had the same solid background, was often used to create the illusion of transparent, anti-aliased images.