Here is what works for me. Go two icons to the right of the smiley face and hover over "Attach Image/File". Left click once, and click on "Browse". Navigate to the folder that contains your image, click the image once, then click "open", and it should show up below the comment box on the left.
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https://www.dropbox.com/s/pfivofjmliewhb6/IMG_20180531_211515538.jpg?dl=0