hey, i was wondering if someone could point me in the right direction for how to create an effect where when a user clicks either a left or right button the pic in the background gradually morphs into the next pic using a effect where sweeping pixels move across the pic and in their wake reveal the next picture.

I was thinking of using masks etc but i don't want to start and find i have chosen the completely wrong road.

a (very quick) mock up of what i'm thinking of is at
http://www.pixelbypixel.newfreehost.com

any help at all appreciated.