The discovery of "zoom" mode for TOPAZ ebooks (
Kindle's Sort Of "Zoom" Feature) turned my attention to images in MOBI/AZW ebooks on the Kindle. These do not have the zoom mode, even though an "image" mode is standard with MobiPocket Reader on most device types. However, small images are always doubled in size. This does not happen on other devices so far as I know, but it seems to be a good idea given the Kindle's screen size and the fact that very small images are common in older MOBI ebooks. On the other hand, complaints about the quality of images in AZW ebooks may in part be due to the small size of the underlying image. It would be much better to start with a larger image.
I attach a test MOBI file, and screenshots from a Kindle and from Windows MobiPocket Reader with a 600x800 reading window. The treatment of images is dramatically different on the two devices. Under Windows, all images are displayed full size if possible and if not an "image mode" is available by clicking on the image. The image mode allows magnification, but the standard view is always unmagnified. On the Kindle, all images are doubled in size up to a maximum of 525x640 and images that are then too large are resized while maintaining their aspect ratio to fit. Unlike with TOPAZ ebooks, there is no image/zoom mode. In previous threads (e.g.
All on the Mac: PDFs with Formatting) I had concluded that the Kindle's MOBI/AZW maximum image size is 520x640. Direct inspection of the attached screenshots indicate that it is actually 525x640.
Amazon DTP software forces images to be no larger than 450x550, see
Full-screen Images. So all its images will be resized on the Kindle! However, its recommended aspect ratio of 9 to 11 is very close to the actual aspect ratio of 525 to 640.
The MOBI ebook contains a few more details, which I won't repeat here.