How do you make a picture change from 1 pic to another repeatedly?

i think the format is '.gif '



in photobucket and in myspace there are pics that change from 1 pic to another over and over again....



i want to make my own,



how do i do it?where do i go?How do you make a picture change from 1 pic to another repeatedly?
you need a gif animator



http://www.gif-animator.com/