Sheri, when you are logged in click the photo to go to its page and below the photo you should see a panel of options titled "Edit Photo Info".
In the top left corner of this screen area you should see "People, Places & Things in the Photo" and beside this a field labelled Add:
Enter the ID of a person (Alexander-2572 in your example) to asscoiate with this photo and then click the SAVE CHANGES TO PHOTO INFO button.