These are the little graphics of Type 3s and Type 4s which can be added automatically when you send a post.
Go to "My Profile" and there's a gallery at the bottom to select an (almost?) appropriate one - or not if you prefer! Click on the spot below the picture then 'select avatar' at the bottom of the page, then 'submit' on the profile page.
It's better than a photo of the person, don't you think?
Find info here on using the site and any changes eg How to show an Image
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