In other words:
1. "POST REPLY" is the thing you do in every thread, whether here or in the groups - this is the way to post.
2. After you know that, you need to get your image from your gallery, and for that, as the others explained, you open your image (in your gallery), and on the right side bar, bellow the "Colors" you see the "Embed". Click on that and copy it.
3. You paste the code that you copied into the space available to you when you "Post Reply"