Open one of the posts on your site where our share buttons are present and right-click your mouse. From this menu, select the option to view the page source. Then, perform a 'Find' in your browser menu to locate the following text:
If you find a matching string, visit the Facebook Debugger Tool and input the URL of the post. Look for any open graph errors and check if an image appears in the results. It should match the URL that you saw when you found og:image in your page source.
By using this form, you are resetting Facebook's cache of your page data and prompting them to scan the page again. If an image is present in the results (without any error notices concerning the size), reopen this post in your browser (or reload the post if you still have it open in another tab) and try sharing again. Does the image appear?
If it does, you'll need to prompt Facebook to scan other pages of your site to update their cached data by using the Debugger Tool. If this didn't fix the problem when you shared with Facebook, I'll need to take a look at the post you are testing. Thanks!