You mean like a browser exploit in image? How do you know that imgur can catch those? What about zero-day. Pretty rare phenomenon & many sites show any img. Suppose could be a UI pref.
Tried researching exploits/vulns based on imgs with external sources. Not really finding anything that serious or that hasn't been fixed. Will look into more.
You can't implement a system to block broken SSL requests from rendering images to display? Not to sound anti-privacy, but there are plenty of other ways to track Memo users already.