I want to add an extra file uploading field to joomla 2.5 registration form for uploading the photos of the users :
You have to:
1) Modify the file components/com_users/models/forms/registration.xml
and add extra fields to it.
2) Modify function register($temp)
in components\com_users\models\registration.php
so you can save path / paths to your file / files.
3) Maybe add additional table in your database or add columns to #__users
table