I thought photos on
https://i.stack.imgur.com/ weren't meant to expire? Or at least, not expire for a very long time without views?
It seems strange that two images on a question are gone, but one survives...
Imgur's FAQ mentions image-retention here, and basically says 1 view in 6 months will keep an image around. Though there is possibly a different "policy" in-place for i.stack.imgur.com, as they have/had a "Pro" account.
Though this question is two years old, the most recent outage was October 4th, 2014, and only affected the v2 API (see here)