Images are especially good for caching (but you need to make sure you’re not sending from your server directives not to cache them!), and thus, cloudflare
But even if your images are cached - if the HTML you’re serving is never cached, for the loading of the HTML itself (and not all subsequent images), Cloudflare will have to go to your server, at least for the initial HTML. And if THAT is slow, users might not like the experience. Ideally if you don’t have customized content (or at least, for not logged-in users), and all users get exactly the same page, you should make it cacheable.