LIT is usually the best choice, but recent MOBI versions typically have images about as good a LIT. In this case, Amazon has a free sample for the Kindle (AZW/MOBI). Based on the 1st chapter I would not describe this as "extensively" illustrated, but some of the images are a good size. See the attached screenshots. The text image is 486x555 pixels. Note that the Kindle doubles the size of the "MERLIN" image.
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