stacking transparent images

here's what i would like to do - but don't know how to do:

against a non-transparent image (it could be a bg image, or not), place a series of transparent images one at a time in a stack. effect would be to show a cumulative result.

my particular problem: against a map of the Atlantic, show ship voyages during the eighteenth century. i have all the data i need, and i have the transparent images for each year. now i want to let the user select start/end years and watch the voyages accumulate. if anyone cares, i used the open source stat package R, and the data available from the CLIWOC project.

http://forums.runrev.com/phpBB2/viewtopic.php?t=3353

Few days before i was facing the same problem.While searching for it, I got a topic.It helped me a lot for solving my problem.Here I have given the link.You also try.

?? I read the post but I just don't see the relevance to my problem. Could you give me just a bit more of a clue as to what to do?

Have you tried putting a z-index and have each img the same x and y coordinates?