'Tagging' a photo is a way to indicate who is in a photo on Geni. You can tag a photo while viewing that photo or during the photo upload process.
To tag a photo that has already been uploaded to Geni, click the 'edit' link next to 'In this photo' under the photo. Start typing the name of the person that is in the photo to select the perso with the people tagger.
Repeat this process for each person in the photo. When you have added the name of each person in the photo click done.
This photo will now be associated with each person you tagged. If this is the first photo tagged with a person's name this will be their profile photo. The most recently tagged photo for each person will be displayed on their profile.
Related link: How to Upload Photos on Geni.com (YouTube video)