I’d like to have ability to resize pictures on my site.
Algorithm:
- You upload an image to site. (grey)
- That image appears in orange part.
- Red part – is a rectangular, with help of which we can choose which part of image to show in preview_zone.
- After I choose right zone, I press OK and data(resize zone) uploads to server.
- Server crops image with PHP.

Maybe you could find any articles, how to do it, or sites with such api.
Or do it by yourselves.
I would be very gratefule for your help.
You should check out JCrop jQuery image cropping plugin:
http://deepliquid.com/content/Jcrop.html
Repo: https://github.com/tapmodo/Jcrop
https://github.com/tapmodo/Jcrop/tree/master/demos
Examples: http://deepliquid.com/projects/Jcrop/demos.php
One of the demos seems to match your requirement. (http://deepliquid.com/projects/Jcrop/demos.php?demo=live_crop) You can make small modifications to get your desired output.