How do you handle that users might embed external http images in a page? Can't you somehow warn about this during editing? Asking because that's the problem I

How do you handle that users might embed external http images in a page? Can't you somehow warn about this during editing? Asking because that's the problem I see at my site currently (https://groni50.org https://groni50.org). In this case I'll just upload the external images to our site. I'll also brief our users. But I wonder if something couldn't be averted/checked in the wysiwyg editor.