Use the
Facebook URL Debug
tool to scan the page you're trying to share. See if there are any errors there. Warnings are generally 'ok', but should be investigated. Errors mean that your tags likely aren't being used at all.
That should show all of the images that are being detected through Open Graph settings.
If that doesn't help, or there are other issues it brings up, let us know what page you're trying to set the tags on and we can investigate further.
Thanks,
Alex