How to Fix Thumbnail Image Not Showing On Facebook
If you have changed the Share thumbnail and you are still unable to see the right thumbnail being shared on Facebook or other Facebook related products; this is a known issue and is mostly because of the caching that Facebook does .
To fix this follow these steps :
Add the link you are trying to share there and run the debug tool a few times. This might take a while but after 4/5 tries and 5/10 minutes of waiting the new thumbnail will appear.
I know this is frustrating but Mark Zuckerberg can be a bit tedious to deal with :)
Here are a few more detailed tutorials if this does not work for you .