they are stored in a database, which i believe (correct me if im wrong) only save info where the image is on your phone (photos app) and then when someone calls it goes to that photo. the database is in/var/mobile/library/addressbook/AddressBookImages.sqlitedb
that is the file, if you know coding you may be able to change it, you could just use an app videoringtones (not sure what it is called), then just make a video 30 seconds long of a picture.