@themetman - I don't have an answer for you, but I'll brainstorm a few thoughts/questions that may or may not be helpful. Maybe, if you provide additional information - we can offer more suggestions.
1) How are you placing the images on the page? It looks like you've added them to the body field. Are you using the image widget in ckeditor?
2) Do you have any image fields on your nodes - is it different for images in the body field from images in an image field?
3) Are you using services like Cloudflare or other server based caching that might be overriding the Backdrop cache settings?
4) If you edit a node text and image, does the text update but not the image? Is this problem only with images or is other content effected?
5) Have you ruled out browser caching as the problem. If you open the site in a new browser, in an incognito window, or on a new computer - does it server the new or the cached image? Have you tried clearing your browser cache?
6) While views and blocks can have their own caching issues, it looks like your images are in the body field of a node page, so they should not be effected by either of those options. Clearing the Backdrop cache is good, but you've done that (hopefully more than once).
If you figure this out, let us know what you find. Otherwise, proving more information might be helpful.