I'm nearly finished a 3 month Flash project but now the most mystifying task is
at hand, left until last. The short of it is this: Clients will be uploading
jpg images, via a Flash/php interface, to a web server. I need to ask them to
provide a thumbnail size image (no more than will fit within a100 pixels
square) and also one that will fit within a 450 pixels square area -- i.e 450
pixels on its longest side. My more web savvy clients will have little problem
in doing this. But wouldn't it be nice if all they had to do is submit a single
jpg image of any resolution (beyond a minimum size) and have a "slicing/dicing
compacting machine" do the work for them and store it (via php?) in their own
folder. I don't think Flashcan do this and not php alone. I'm looking for the
simplest way out and don't want to buy into complicated software that do this
job and other things way beyond that I don't and won't ever need. Any
suggestions from the audience??
thank you..

It would be possible to do it in flash (as3 in particular)... but it really
doesn't make sense to unless you want people to choose a specific crop of the
image for the thumbnail creation.
You probably really need to search for a php script that will serve the
purpose. I'd imagine there are some to do this. Try googling for "php
autogeneration of thumbnails" and take a look through some of those links... (I
didn't look through them, but it seems like the type of thing you're after).