I wrote a uploader in flash for our photo hosting website many years ago using alchemy. From memory I wrote a wrapper for libjpeg that could do the resizing in C which was then compiled into flash bytecode. That could then be imported into the swf that would handle multiple files drag-drop resize / upload. That all interfaced with the page through some sort of JS bridge.

I'm amazed any of it worked to be honest, somehow it managed to chug along for a decade without issue.