A cool tut about marbling images!
http://www.taron.de/forum/viewtopic.php?f=7&t=71&p=2338#p2336 by Borhani59
(French translation http://www.3dsaloon.fr/phpbb3/viewtopic.php?f=12&t=3990&p=56610#p56610)

my little try :)