You’re asking about oneboxing, but as I said:
If a user directly links to an image from one of the services you mentioned:
There is no onebox.
Plus, Oneboxes are cached for 24 hours, I’ve not seen clarification either way as to whether unchecking that box disables the caching of the image asset explicitly, I’ve only seen it employed for image embeds. I guess it’s something you can go off and test though.