I don't own a large tablet, so I was slow to realize that Amazon's 8.9 inch Fire HD was seriously degrading the images in Kindle editions. (The same doesn't seem to be true of the Apple iPad.) The problem arises with the larger, high-resolution screen, combined with the lack of a software default that would enlarge an image to fill the screen, or something close to that.
For years Amazon has recommended that we use images sized to a 600 by 800 pixel ratio, and that is what I have used in recent years. But even when the image was smaller than the recommendation, it was enlarged to full screen in most Kindles and Kindle apps. With the advent of the Fire HD tablets, this no longer seems to be the case.
As a fix, I have made two changes. First, I now upload images that are 800 pixels wide by however tall. (Some formatters use 1000 pixels wide, but I think the smaller images expand quite well. Second, I changed the html for my images to include a width="100%" instruction. This solves the problem on the Fire HD and on its larger sister, which I think of as The iPad Killer, and it makes no difference whatever in the e-ink Kindles or the various Kindle apps. This is what the html looks like in the case of the Sony Data Discman illustration early in my Guide:
<div class="image" id="disc"><img alt="Sony Data Discman" width="100%" src="discman.jpg" /></div>
The "class" tells the Kindle or app to follow the instructions in the style sheet for formatting an image. The "id" links the illustration to the table of contents. The "alt" tells the Kindle text-to-speech feature what to say when it encounters the illustration. The "width" instruction provides an enlarged image in the HD tablets. And the "src" (short for source) points to the image file itself.