Click on the image to go to it's edit page.
Scroll down below the image, and on the right side in the section called: "Edit Image Details", then lust below that is: "People & Things in the Image", there's an option "Remove from Image". Click that for each person linked to that image that shouldn't be there.
If no one is attached to the image, then after 60 days, it will be deleted from the server.
