Facebook caches (saves in memory) the photos in posts as a way to be more efficient. Unfortunately, this may create a Facebook caching issue that Full Frame does not have control over. The Facebook Sharing Debugger is the tool provided by Facebook as a way to delete the saved cache and update your post. Simply copy and paste the URL and click the debug button then on the newly loaded page you can hit the scrape again button to force facebook to scrape the website again and refresh the header image.