The old addon mentioned above only works for archetypes and is using an event handler to extract the image and create as a real Plone iomage object. It could be implemented for Dexterity as well, but I would recommend to implement it as portal transform, like the resolve UID works. This way it would never get saved into ZODB as base64 which is a big win, as I mentioned earlier in this thread.
I did some testing recently with the TinyMCE support for that. But no matter what i did, i get always a src with a link to the external website if i try to paste in from other websites. If i try to past in from local documents, it is stored as a blob: link, which probably could be used to get the image and store it as Plone image. But for external website's it seam that the browsers moved on to not supporting data:image. The paste_data_images flag was true for TinyMCE. You can set this in the advanced tab in Plone as JSON config. But even outside of Plone i tried it, but could get a data:image entry.
Try it here: https://codepen.io/MrTango/pen/rPxBjK
For external websites it, might be only interesting for migrating content purpose. Maybe we could add paste_preprocess or paste_preprocess handlers and mark them at least, so we could catch them later in the backend.
Alternative we could support local images by integrating the powerpaste plugin with powerpaste_allow_local_images enabled.