This one unfortunately I won’t be able to help you because this relates more to the social networking scripts that captures the images. You might want to post this question and issue in the wordpress.org forums and see if anyone has a solution or why that doesn’t happen. Images are put into WordPress posts and pages either as a “Featured Image” or directly inserting one into your content (both are WordPress functions).
Maybe contact support of those social networks and see if they know why.
