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.