Currently on reddit if you attempt to link to a image directly outside of reddit and somebody clicks it, it'll redirect them to a media viewer page that hides all the comments but provides a link to view them. As much as I hate that redirect, I don't think it is a terrible idea for Lemmy to do as well because of the issue your friend had with the thumbnail that you wanted them to click.
That would be great. I am not sure if it was RES doing it or it was a default reddit thing but I do remember images automatically expanding on there. Having it happen automatically on Lemmy would be great as long as the user is not on a slow or data-capped connection.