Go to your settings page and add an COVER IMAGE URL there.
https://steemit.com/@uniquegem/settings

If you don't have your photo on the web, you can upload it to steemit.
Just create a new post or comment and insert your photo there per Drag'n'Drop to upload it to steemit. Copy the generated URL and insert these URL as the COVER IMAGE URL in your settings page and click UPDATE.
You don't have to post the created post/comment, you can CANCEL it.