what you're taking about isn't an easy task, and requires more than one technology. Generally, uploading an image is done with a server side scripting language ( so php, or asp, or jsp, or whatever ). Then some other script to output an xml file. Then load the xml file with the uploaded images into flash.
Alternatively you could use flash remoting, so something like
amfphp
once everything is loaded in, the image sizing within flash should be easy enough.
The last part ( the saving, I'm assuming you want to save the resized images ) is pretty damn tricky. But
some have succeeded through the use of the new bitmapData Class in Flash 8.
Again, not a simple run of the mill project.
Good luck!