Try running your gallery page's URL through the Facebook Sharing Debugger: https://developers.facebook.com/tools/debug/
You could try their Open Graph Object Debugger, too: https://developers.facebook.com/tools/debug/og/object

This should refresh Facebook's cache of your web page, including Open Graph tags (although it may not be instant).

Here are a few other suggestions I've found on the web which may or may not help (but might be worth trying).

(1) Try placing your og:image tag before all others on your web page.

(2) Try using og:image:type, og:image:width and og:image:height Open Graph tags in addition to your og:image tag. For your image, try the following:

<meta property="og:image:type" content="image/jpeg" />
<meta property="og:image:width" content="1024" />
<meta property="og:image:height" content="730" />

(3) Try renaming the image you use for your og:image (and the corresponding og:image value itself) so that Facebook has to fetch the 'new' image.

Hopefully one of these suggestions will help.

You're welcome.
I hope one of the suggestions works for you.