The reason you have this image problem is probably because formatting images smaller than 100% screen width is done differently for KF7 and KF8 devices. If you format images for KF7 then you must use the actual image dimensions in pixels. But if you are formatting images less than 100% screen width for KF8 devices then you have to format the image dimensions as a percentage of screen width(not in pixels). And the only way you can resolve your issue would be to manually dual format all your images for both KF7 and KF8 using media queries in the epub stylesheet.
But if you want a much easier and quicker way to autoformat your smaller images to display correctly on both Kindle KF7(older Kindles) and KF8 devices then you should download Sigil (epub editor) from the internet and install it. Then download my Sigil plugin called AddKindleMediaQueries from The Sigil Plugin Index here: https://www.mobileread.com/forums/showpost.php?p=2940034&postcount=1
Then do the following:
1. Install the plugin into Sigil. See the AddKindleMediaQueries plugin release notes for how-to details.
2. Export your ID doc as an epub to your desktop.
2. Then load your epub into Sigil.
3 In Sigil, click on Plugins > Edit > AddKindleMediaQueries, which runs the plugin.
After running the plugin, all images that are smaller than 100% screen width will automatically be re-formatted(dual formatted) to correctly display on both KF7 and KF8 devices. All this plugin does is automatically create the relevant media queries in the stylesheet, which will act to format your smaller images in the correct way for both KF7 and KF8 devices. There is no html coding needed -- its all done automatically when you run the plugin.
I have to confess, however, that I don't use media queries. I size my "sorts" (fluerons, colophons, whatever) on the small side, so they don't blow up full page on the obsolete Kindles. I figure I'm doing their owners a service, pointing out that it's high time they upgraded. I'm sure they're not using CPM computers and probably not even a dumb phone (I have to tread easy here, because Susan uses just such a phone), and I have zero pity when a brand new Fire tablet can be had for $49.99. Crikey, there are BOOKS that cost more than that!
(Don't trust KDP to publish a print edition. Don't trust CreateSpace to publish an ebook. Each does one thing well and the other thing poorly.)