I'm not sure if WordPress is actually doing this, but they might be using something like camo[1] to transparently rewrite any http:// URLs to an image proxy running on SSL.
This gets harder to implement correctly depending on what kind of content you allow on your sites (i.e. does your CMS only permit sanitized HTML, or are users allowed to do basically anything?), so it's not a perfect solution for everyone, but it might work here.
This gets harder to implement correctly depending on what kind of content you allow on your sites (i.e. does your CMS only permit sanitized HTML, or are users allowed to do basically anything?), so it's not a perfect solution for everyone, but it might work here.
[1]: https://github.com/atmos/camo