Plugin takes featured image from the post. if there is no featured image it will take the first image from the post body. if your post body has no images it will take the specified default image. if default image is not specified it will take first image from the page.
Please set featured image to clearly define what image you would like to show or specify Plugin Settings->Other Settings->"Open Graph" Tags->Default Image URL for og:image tag for the default image.
Image must be set at the time you hit the "Publish" button. Some third party plugins that make posts sometimes publish post without featured image and then add it later. If this is your case, you can try to delay the autoposting for several minutes.
Some networks (like Google+ and Stumbleupon) could ignore the image you send and take the image specified in the og:image tag, so please make sure this tag is set as well.