Thanks for calling that out. It looks like attaching images directly to a comment only works for kbin instances. This is what it looks like from kbin.social. I just tried viewing this thread from lemmy.world and the images were not showing up.
To be honest, I don't want to go through the effort of editing my comments to correct it right now. But in the future I'll go back to hosting images and linking them in my comments, so anyone from any instance can see them. That's a shame, because attaching images to comments in Kbin is super convenient. Oh well! Thanks again for letting me know.