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 adds these meta tags to each one of your posts, so you do not have anything more to do.
We’ve even made some improvements to the generated Open Graph tags in the latest version of Jetpack, so you might want to update.
You can see what Facebook sees by entering one of your posts’ URLs in Facebook Debugger:
https://developers.facebook.com/tools/debug
You’ll notice that an image has to be at least 200x200px to be used by Facebook. That might explain some of your issues, since some of your podcast images are smaller than this requirement.
Once you’ve generated the tags (through Jetpack or another plugin), that’s up to Facebook to use them to generate the post preview, or to ignore them.
If you’ve added images, if they’re large enough, if the debugger doesn’t return any issues, and if Facebook still doesn’t generate a proper post preview, I would suggest that you open a bug report here:
https://developers.facebook.com/bugs
I hope this helps.
