For what it’s worth, in a recent post (What's In Your Bag? - #44 by anarcat) I have done the “drag and drop” thing in the hope it would do the right thing, but the “thumbnail” was also that huge file. I have since then re-uploaded a 1080p-maxed picture to reduce the pain.
It would sure be nice to have the full resolution images available on click while keeping threads with more reasonable thumbnails… I haven’t looked at discussions on discourse on the topic, but it seems to me this should be fixed (or reported) upstream! 