I think you may have misunderstood the issue, or maybe you are seeing the cause of the bug 
I did not upload that image to Discourse, I just pasted a link, exactly like @Canapin 's screen recording below 
It’s been pulling large images for years now @Canapin mate, many years, it’s only stopped working in the last six or eight weeks.
Thanks for testing 
I can replicate it on our forum, on try.discourse.org and on meta.discourse.org
I’ve placed a bounty on the issue:
Thanks again to everyone who has looked at this issue so far 