There really isn't an easy way to this in as2. You would have to loop all the pixels. Also, in flash, we would only be talking about saving it in a Shared object, not really saving the jpg file for instace. You would have to send it to the server side, and parse the data there to really save it.
If you can get your hands on flash cs3 or newer however: http://ask.amoeba.co.in/save-images-...ve-jpgencoder/