%IMG% is useless for Facebook. %IMG% does not add an actual image, it adds the link to the image. THis tag should be used for social networks that support HTML, so you can wrap it in tag and show the image.
Facebook does not support HTML, so %IMG% is useless for it.
You should use â€˜Image postâ€™ if you would like to post an image or â€˜Attach/share your blogpostâ€™