The tutorials posted are what you want. Just instead of doing it on normal text, do it on text with a clipping mask applied. To do this, create a new layer, then go to Image -> Apply Image. Then right click the applied image and hit Create Clipping Mask. Move the clipping mask around and then follow the tutorial.
http://forums.d2jsp.org/index.php?act=ST&f=246&t=6319265&st=0#entry32939629