My browser is reporting mixed HTTPS and HTTP content on the main site because the "Image of the week" in the header is HTTP:
<img src="http://i.stack.imgur.com/hJKvB.jpg" alt="Random Flower in Lawn">
Given that the rest of the imgur user-provided images were changed to HTTPS-only, it is obviously an oversight that whatever form the moderators have to edit the image of the week allows non-HTTPS URLs.
I'm tagging this as support because the immediate problem is the image right now, and it will probably be a sufficient work-around for the mods to add the URL protocol to their mental checklist for updating the photo. Besides, since they know what the actual interface is, they're better placed than me to file a bug or feature-request.