You can't add or change the photo of a contact because Yahoo! allows each user to control the photo they display for their own account.

Display photo controls:

• You can't assign a photo to a contact: You can't assign or modify a photo for someone else's account, even if they are in your contacts.
• Your contacts control their own display photo: If someone has their account set to display a photo, then that is the photo that you will see for that contact.