The Kindles are among the best EInk readers for images in ebooks. It is true that AZW images tend to be smaller than the same image in an ePub, but with most ePub-based EInk readers whatever configuration the publisher has decided on for the image is what you get. There is typically no zoom option (although flipping to landscape may change the zoom).
The Kindle Touch and probably the non-touch (but not the Kindle Keyboard) is likely to get the new KF8 format
"soon". This is essentially equivalent to ePub. I don't know if Amazon's KF8 support for the Kindle EInk devices will continue to include the image zoom support though. Newly published AZW ebooks from Amazon are delivered today as KF8 to the Kindle Fire, K4PC and K4Mac and delivered as MOBI to other apps and devices. Once it is available on your Kindle you will be able to redownload such ebooks as KF8's.
If you strip the DRM (if any) from an ebook, you can replace images and make other customizations. This is easiest to do in ePub, but if you start with a MOBI (or KF8) then Calibre can convert them to ePub, make any changes with an ePub editor, or Calibre's "tweak ePub" option, and then convert back to MOBI.