How do I let a user paste an image on a webpage?

Question

I want to give the user possibility to paste an image into a webform. I don't need to display the image, all I need is the image's location.

I know it's possible because the guys at CKEditor(and other editors) are doing it.

If you go here, http://ckeditor.com/demo and paste an image you copied and then right click on it and go to image properties you'll see they have the image's address.

How can this be done?

Thanks