Here's a bit of an esoteric issue that was a bit tricky to hunt down. I hope that this blog post helps the few people out there that it applies to!
You've got a secure (https) Drupal site, and you'd like it to contain facebook-compatible, open graph meta tags. So, you download the meta tags module and do some configuration magic. Everything looks good until you post a page on Facebook, and the image doesn't work!
Ok, so let's not serve our og:image tags over https. A quick theming hook will fix that in no time: