Update March 2017: Our Benevolent Overlords are making this change for usOur Benevolent Overlords are making this change for us (without bumping posts), so I'd now fairly strongly say this kind of edit is now not acceptable.
Original answer from November 2016:
My take: this is just about on the limit of acceptability. Noting that:
- Imgur (which Stack Exchange uses for its image hosting) supports HTTPS perfectly well.
- If you upload an image today, it gets an HTTPS link.
- HTTPS is a good thing in general.
I wouldn't object to this happening as part of a larger change to a question, or as an occasional change to a particularly notable question. Bumping 15 questions to the top of the active list wouldn't be so good however.