Facebook uses the Open Graph information on your page or post information to decide what information to pull when your post is shared.
Normally you don't ever have to care about what Open Graph is, but how it's set depends on your theme and plugins.

Some themes only apply Open Graph tags to the featured image and identifies the title and description of the page as the information to be pulled when your post is shared. If you are given a number of pictures to choose from, that most likely means that the Open Graph tags are not defined properly.

If you use a plugin you can define what pictures are to be used and sometimes you can even specify a separate image for each social media. Some plugins even allow you to create a hidden vertical image that is only pulled when your post is pinned at Pinterest.

But what if the Facebook debugger doesn't debug?

Try out these solutions:

• Clear out any site caching plugins you may have installed to make sure the bots aren’t getting a cached version of your URL.
• Make sure you saved your content after you edited.
• Check your warnings and errors in the Facebook debugger. Sometimes if your image is too small or too large, it won’t be recognized.
• Is your site publicly accessible? If you’re using a “coming soon” plugin or maintenance mode is active, Facebook won’t be able to see your page. The same if your post is in Private mode or on a scheduled publishing time.

If it still doesn't work, them I'm sorry to say that I don't know what to do.

Thanks for reading, and I hope this did help you out.

PS
This was my first tutorial ever publised here at WA. Please excuse my poor educational skills :)