Photo editing site: Editing low res and save hi res

I need to know level of difficulty/hours needed to do this:

A user uploads a photo into an editing tool. It takes a couple of seconds to upload the full res version. Then, when the photos are being edited in the tool (cropping, color, etc.) it converts to low res for faster editing, so the high res image doesn't have to load every time there's a change. Once the user is done, and/or adds their final product, the manipulated image is saved to hi res again, for print.

What is the time estimate/level of complexity if this? The code is there for the tool (some advanced JavaScript), just need to change the speed of editing. Server disk write speed is plenty fast. Amny comments would be appreciated.