In July Facebook made several changes to their API interface, affecting posts shared on profiles, pages and groups. One of these changes was, that Facebook supports individually selected images for images post format only. For link posts Facebook will display images set in the "og" (open graph) parameters of the given blog post. If you have set og parameters for your featured image in your blog post, Facebook will display this featured image. Otherwise Facebook will display a random image from your blog.
Learn more about "og" tags in our FAQ: What are open graph tags and what do I use them for?
This change does only affect link post formats, not image post formats. If you share your blog post as image posts, your selected image will be displayed in the shared post on Facebook.
You can select the post format for link posts or image posts as default settings for your networks in the Blog2Social settings.
We will provide a solution to fix this issue with the coming update in August. So with the next update your selected image in Blog2Social will be display on Facebook for link posts as well as image posts.