Hi!
The Xinha editor has this feature build in. Users can upload images directly from a cam and when inserting the picture in a page the user just has to give the desired width of the picture an xinha renders a resized copy of the original picture.
I think this is a great feature - this way one can upload the picture in original size and use it in different sizes later on.
Unfortunately Xinha isn't available at the moment as a module. I really would like to have it back...

Alex