[@Altered Tundra]

Right, that would be because the Guild allows people to embed images that may come from insecure sites. The Guild itself is safe, but that warning can't be modified without modifying the site to host all images locally. If you want to prevent that when linking an unencrypted message, you can upload images to your profile, where they will be served over SSL/TLS.

[@NuttsnBolts]

The entire site uses SSL/TLS, but it's a byproduct of the Guild allowing people to embed images without uploading them.