This is a problem which might occur while sharing contents from our site.
Even though if we clear cache from our browser / from our site, still facebook uses the old thumbnails.
To resolve this issue we have one best solution at facebook level.
If in case the above solution did not work, please try adding "fbrefresh=ANYTHING" at the end of your URL and follow same steps.
NOTE: Mostly param fbrefresh might not be necessary as the debug tool refreshes the object without it.