As of July 17, 2017 Facebook will auto-pick the image for your Social Share posts. The recent update to Facebook’s API is their effort to reduce sharing false news. Constant Contact is actively working on a solution that will enable you to customize images the way you’re used to.
In the mean time, if you want a certain image to show up in the post, the likelihood that Facebook will select it is increased when you include it in your email or coupon. Other social networks, such as Twitter and LinkedIn, still allow you to customize the images in your posts the way you’re used to. While we work on a solution, when using Social Share, consider creating two posts – a post just for Facebook and a second for the other social networks.