However, when the Facebook link is selected on others there is a message that always says "Sorry, this feature isn't available right now: An error occurred while processing this request. Please try again later".
Having Googled the error message, the two most likely causes seem to be:
(1) It may be a temporary Facebook database or server issue which should hopefully resolve itself soon.
(2) It is a browser cache issue which can be solved by completely clearing your browser's cache and cookies.
If the problem is #1, then there is little you can do but wait until Facebook resolve the issue.
However, if the issue is #2, then completely clearing your browser's cache and cookies (or trying a different browser) should hopefully help.
Also there appears to be no definitive pattern as to which picture shows when the Facebook link does show one. Most times, it is the index picture. Some times, it is not.
Try running your gallery page URLs through the Facebook Sharing Debugger: https://developers.facebook.com/tools/debug/
You could try also try their Open Graph Object Debugger, too: https://developers.facebook.com/tools/debug/og/object
This should refresh Facebook's cache of your web pages, including Open Graph tags (although it may not be instant).
If you continue to experience difficulties, please post the URLs to your gallery pages so that I can take a look at the problem for myself and hopefully help further. Thank you.