if I understand correctly, you have a profile that you entered a photo for, and made it the profile-photo, and NOW you want to REPLACE the photo with another one?
This SHOULD be pretty easy.
1) Add the other photo. Open the photo in your album.
2) Identify the person in the photo, either by entering their name (if a close relative) or by tagging them ( http://wiki.geni.com/index.php/Typical_replies#Tagging ).
3) Press the "Set Profile Photo" button.