Yes, I understand what you mean but to be honest, i do not like this method with ajax. It creates a tempory files on the website and if the user stop the action, the file does not disappear (with my tests). It would create a cron file to remove the images in the temps folder etc... I find this really paintfull method ^^
I found it easier to ask the user to change the thumbnail if he wanted but only after sending the file and the creation of the first thumb.
Added 2 days later:
After some research, I found a script that launches the client-side script.
Here it is : http://marx-tseng.appspot.com/image_crop/index.html
Now I find myself with a new problem, I can upload the big picture with <input type="file".....
do you have a solution to upload the second image in an input format (gif,jpg,png) and not base64 ?