Missing images on HTTPS? They have to be HTTPS too

A couple days ago, I noticed that Blogger allows HTTPS access to blogs, so I turned that on in a hurry. The HTTP version doesn't redirect automatically, but HTTPS started working, with the exception of the Lojban Stack Exchange site proposal ad on the right. The image was not appearing at all there.

The problem was that the image wasn't served over HTTPS. Most major browsers I'm aware of require secure pages to load only secure content - even if that content is just images. (Of course, unsecure pages can load secure content.) Updating the URL of the image to the HTTPS version fixed the problem.