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 .