After further investigation, the problem appears to be linked to a Facebook bug, at least for some of you.
When you share a post on Facebook, or when Jetpack Publicize publishes a post to your Facebook page, Facebook crawls the page and looks for Open Graph meta tags in the head to build a complete post preview (with an image, title, description, …).
Jetpack automatically creates these tags for you, unless you’re already using another plugin to manage Open Graph Meta tags.
Could you all try the following:
- Copy one of your posts’ URL.
- Paste it in Facebook’s Debug tool, and submit: https://developers.facebook.com/tools/debug
- Your Open Graph Meta tags will be listed there, along with errors if Facebook experiences some issues when looking for these tags.
Some of you will notice that Facebook returns the following error:
Provided og:image is not big enough. Please use an image that’s at least 200×200 px.
This may appear even when the image is larger than 200x200px. If that’s the case, you will want to comment or add a vote to that Facebook Debug, to let the Facebook developers know about the issue:
https://developers.facebook.com/bugs/483025408453290
Thanks!