Per Roadmap to HTTPS: serving and uploading HTTPS-images only, we need to help out to convert non-imgur http images to https. Assuming the images still exist, this is just trivially uploading them to imgur.
Specifically, when you edit the post, the "bad" http image links will show something like this in the preview where you can just click the upload link and follow through:
If that doesn't happen, most likely because the image no longer exists, we have to try to find a copy somewhere else, maybe via archive.org.
If you'd like to help, just start going through the list in the answer, and edit the answer to mark things completed when you finish them.
Yes, this is going to cause a lot of churn on the front page, but things are broken, so we have to get through it somehow :( Other sites are doing the same.