Cropping images with html2canvas

I have created a plugin where i can upload picture to a frame and position it with drag and drop and then click a button called buy to crop the positioned image but here is the problem. The image container has a CSS property of overflow:hidden so when i try to crop the image only the visible parts gets shown but when i drag the image it doesn't show the rest of the image instead it shows white pixels.

You can see it live here on my website: