Once a post or page has been shared on Facebook, the images have been stored in a cache on their server for future shares. Facebook doesn’t look at the page again for new images until their cache expires. This time period is unpredictable, but can be days or weeks. You can force Facebook to update its cache for your post/page by entering the link to the specific blog post you’re sharing into their Debug tool on this page:
After pasting your link into the box on that page, click the “Debug” button. Then try sharing your post/page on Facebook again.