Most social media platforms use your featured image as a preview image when you share a page.
Unfortunately, Facebook is one exception.
They developed their own methods to identify content, which almost fail, unless the pages were set using the right tool.
In this case, the right tool is Yoast SEO plugin, it - even the basic version will suffice, and it is included in every AgentFire website. It will allow you to set a custom preview picture exclusively for Facebook, as well as a custom title and description for Facebook sharing.
How to set custom Facebook preview image for your pages
This process applies to every Page, Spark Page, Area Page, Convert Page, Search Page or Team Member Page, the only exceptions are Blog Posts which have the "classic WordPress editor" with Yoast SEO directly under the content box.
First of all, you'll need to open the page in the editor.
Click on the 4th icon with a letter "Y" on the far left side to open the Yoast editor in a new tab.
Scroll down until you see the YOAST SEO box.
Click on the 3 connected dots; Social Sharing Icon just below the Traffic Lights Icon.
Here, you can customize your Facebook post's title, its description and most importantly, you can set the preview picture.
Simply upload a new picture or select an image from your Media Library.
Select a picture which is approx. 1200x630 pixels, and click the "Use Image" button on the bottom right-hand corner.
Select the Update button at the right to save your work.
And that's it, test your work by sharing the page on Facebook ;-)
Steps for Blog Posts
For blog posts, the steps are basically the same, the only difference is that you don't even need to click on Yoast's icon at the visual editor. Simply navigate to your Blogging section, and click on the Edit button beside the Blog post you would like to edit.
This would lead us to the "generic WordPress Editor", from this point on the method is exactly the same as is outlined above.
If the preview picture would not get updated automatically in an couple hours maximum use Facebook's own Sharing Debugger to get Facebook to crawl again your page's preview picture https://developers.facebook.com/tools/debug/