The early Kindles enlarged every image to full screen (unless it was very small to begin with). I thought this was still true of e-ink Kindles, but perhaps it's not; my only one is a K3 Keyboard. Starting with the Fire tablets, however, screens became hi-rez and images were shown actual size. (Though I've been told that a Pixel Is A Pixel Is Not Always A Pixel when it comes to LCD screens.) A double tap will zoom them to full size.
I had your challenge with one book--there was simply no way it would be legible on the average e-reader. It was a strong horizontal. So I sliced it down to a normal vertical image and put a link below it to the full image online. Of course I don't know if anyone ever clicked through, but the option was available to them if they chose.
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