This add-on specifies <og:image> meta tag by finding [ img] or [ attach] tags in content. It will tell social network's crawler to use that image for the thread.
Besides grabbing <og:image> from data, this add-on allows you optimize images to generate great previews:
1. Specific image dimensions: only [ img] or [ attach] over this dimensions can be used as <og:image>
2. Grabbing x number of images: so you can display multiple images in a FB post with link
3. Limit images' size from grabbing in KB