Hello,
1. To reproduce you should have a group with a profile picture and cover set up.
2. Now share some pictures in group stream (simple upload in stream)
3. Now press social share on an image you just shared
4. When pressing a social share button for first time, image wont load, or sometimes it load just fine.
5. Image just loaded is a group profile picture not the image on which you pressed the social share button, if the image is correct then try to share it on facebook, and in facebook you will notice that it shared a group profile picture.
This problem in not just with groups, whatever you shared on any stream it load something else not that you try to share, i am talking about images.