Capture signature using HTML5 Canvas and save to database as image

I need to capture and save user signature to database on asp.net webform.
I found this plug-in: https://github.com/szimek/signature_pad
I downloaded the example project and it is very simple, it has only fews file but I could not understand.
demo here: http://szimek.github.io/signature_pad/
How do I use this on a web-form page, let say on "save" button click postback event, it will save the imageurl of the signature (which is under this format) 
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAApIAAAE+CAYAAAA+vvBuAAAgAElEQVR4Xu3de6w1XV0f8K8WvIEiFTAKXhBrab0AxUraaFET0xYxSGrrpSbUahsjbcUoXhKtIiaaSFSSojHRUP7w0rQGKbZJ00ahN....etc..... to a string variable so I can save this string to database later? (so I can display this image to a webpage by using src="data:image/png;base ...