By default, most programs that advertise your Wordpress posts will show the first image found, no matter if it's a thumbnail, sketch, or full sized photo.  Situations this can apply to include newsletters, RSS feeds, and social media sharing.

To fix this, always set a Featured Image.  As you are crafting your post, in the lower right hand corner, find the box that says Featured Image.  Click the link and choose the image you want to appear.  This image will be chosen in the above situations.