Facebook uses Open Graph (og) tags and the content on the page to render the title, description, and image shared. In some cases when there is no main image on the page or to many images like the home page, Facebook may scan the wrong image.
You can use the WordPress SEO by Yoast plugin to set a specific data for Facebook. When creating a post or page, scroll down to Yoast SEO meta box below the post editor and then click on the social tab below the stoplight icon. There you will see a button to upload a thumbnail image for Facebook and update the Title and Description that is shared.
Simply add your Facebook Thumbnail image, Title and Description there, and this will fix the issue in most cases. However if it continues to show the incorrect thumbnail, please contact Blackbaud's WordPress Support team for assistance.