this is sort of related, .. but i had the reverse problem and i thought maybe some might find it interesting.
i wanted a user to be able to click on an image and be promted to download. the problem was that most browser force the image to be viewed in the browser, rather than downloaded.
with php, i was able to manipulate the headers of a script, and pass thru an image so it downloads.
http://milanointeractive.com
there is weird things that happen to naming it, but with a little bit of browser detection built in, it will work fine.
i know this doesn't help .. but just reminded me of this project .. good luck =)