Photo editing - hi to low res in editing conversion

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. Any comments would be appreciated.

Um... not too hard, I guess? It is probably strongly dependent on your skill level and how powerful you want the interface to be. For a novice programmer, this could take a few weeks. For an experienced one, maybe only a few hours to a few days.

My general advice is to not redesign the wheel though. There are plenty of free, open-source tools to do this. A powerful (but a little complex to learn) one is the GNU Image Manipulation Program (GIMP). A less powerful, but easier to use, solution would be Paint.NET.

I know your purpose is to cut off time, but in reality you're actually adding more processing to the work load. If your whole purpose is just so that the client-server communcation is lower, then that's another story.

Either way, this solution does not yet exist out there because cropping and any other editing that takes place on a coordinate basis cannot be done on a smaller scale and then added to a full-res image without a loss of coordinate data.