Facebook sometimes take a few seconds to collect a share image. If it doesn't show up when you go to post it try this:
Go to Facebook and paste the share link into a post. If the image doesn't appear it's probably a link you've not shared before. Don't worry.
Refresh the page and try again. This time the image should show up.
If that doesn't work contact us in the in-app chat and let us have the share link to your Publication.