@jhowell...
"Cover Image does not meet the image quality bar of 300 PPI."
I could perhaps put in a plugin resolution check on all epub ebook images. At the moment, I'm not sure that Bundled Python's Pillow module would be able to get the image resolution. I'll have a look and will put in a resolution check if I can do it.
"Cover image occupies 100% of the screen, therefore the image either needs to be greater than 1200 PX width or 1800 PX height."
The above problem is covered by my plugin's dual formatting check. If a 'not dual formatted ' warning is given then you can resolve that by simply running the
AddKindleMediaQueries plugin which automatically dual formats all ebook images for optimal display on both KF8 and KF7 devices.
If you're not bothered about dual formatting your ebook images and prefer just to format your smaller images(ie images less than max screen size) only for KF8 then you can do this buy formatting your smaller ebook images using inline styling and percentage values(which represent percentage of current screen width values). Simple example below:
<p class="image-style"><img alt="" src="../Images/image0.jpg"
style="width: 20%;height: auto;"/></p>
But if you want your KF8 ebook images to be max page size across all KF8 device screens then you can also format them like this(and you shouldn't get errors):
<p class="image-style"><img alt="" class="kf8only" src="../Images/image0.jpg"
style="width: 100%;height: auto;"/></p>
Generally speaking you should also bear in mind that Kindle always prefers the "shrink to fit" approach for adding ebook images at max screen size which means that they prefer using interpolation -- you add an image that is larger than max screen size and Kindle will happily use "shrink to fit" to fit it on the current screen. This is more accurate than expanding a smaller image to fit the page or screen(extrapolation) which can give problems like pixelation. Which is why Kindle usually gives warnings if extrapolation(or expand to fit) is used for max-page-size ebook images.