Until recently, TOPAZ ebook images could be zoomed but AZW ebook images, although potentially doubled in size, could not be zoomed, see
Kindle Automatically Doubles Image Sizes.
The new version 1.2 firmware (see
Kindle Upgrade) adds image zoom to AZW ebooks. Simply select the image using the scrollbar.
It is tricky to get the Kindle's screenshot capability to work with zooms. I found that holding down <alt> and <shift> and then selecting the image and finally pressing <G> immediately after the zoom comes up usually works.
The first two examples are wider than tall, and so switch to landscape for the zoom. Cover pages do not zoom, but sometimes a smaller version of the same image is on the 1st page and it is obviously zoomable. See example 3, which also illustrates than images taller then they are wide stay in portrait mode. The last example illustrates that even small images zoom to the full page size. Note that the original is already doubled in size, so the actual image is 104x56 pixels.
Previously the "optimal" image size was 525x640. I'm not sure that zooming really helps enough to matter for portrait images, but for landscape (wider than high) images it may now be best to use 800 wide images which will be scaled down for the standard page but will show at full size when zooming.