I have published several image-heavy books. Reflowable will cause you endless headaches. At least in my experience a great deal of coding will be required to make it look anywhere reasonable. I just finished converting a print book with 1700 images into a reflowable ePub—no fun. Moreover, if you have an index it’s basically useless in a reflowable epub. Fixed layout epubs will be accepted by some distributors (including the big dog) who won’t accept pdfs. For my money, for anything but phones (typically demands a complete reformat), an interactive pdf is the best solution, albeit it has very limited direct editing capability. For tablets and larger screens I always recommend the pdf to my purchasers. Amazon has a textbook creator that produces their version of a fixed-layout epub from a pdf.
Cheers from someone about to face a winter hurricane.