Hi Martin,
I think what you're after could be done using some sort of script.
Which script depends on you and the resources available to you.

ImageReady (part of PhotoShop) could do something like it for you using javascript on your page. It would be quite simple to do if you have the software, which is quite expensive.

Flash could do it using Actionscript (would be a pretty big file and take longer to d/l than really needed.

Php. I think this may be your best option but I don't know enough about it to advise. You might want to repost the question in the php section (also asp, which I know even less about. lol)