UPDATE: this is happening. See The Big Edit Event (is finished).
Stack Exchange is moving all images from http:// links to https:// links. The ones that can are being done automatically, but there are a lot that are either actually dead or don't have a https:// available. These need to be manually edited.
Here is an SEDE query that finds images that are not from i.stack.imgur. I've been using it on other sites, and have fixed a bunch of broken links, in addition to just switching images to stack.imgur as a precautionary measure, because of linkrot.
There are also tons of questions that need to be retagged. It might be worth waiting until after we decide what to do with our Tolkien tags, but after that, if we're going to mess up the front page to fix images we can retag those too.
Note: Relatively soon, all images linking to a http:// image address will be turned into links.
Rebake remaining posts with HTTP images, so that images linking to HTTP addresses will become links - most of those are dead anyway. This will be a HTML baking change, so no Markdown will be affected, but it'll remove any mixed content even when viewing old revisions.
We should edit before this happens.
What do you think? Should we have a mass editing event?