I added all of the parameters that you could have in the url and made it text boxes on the webpage, so you can update everything in real time. Underneath all of the normal inputs is a textbox that gives you a permanent url so you don't have to keep resetting everything. Enjoy!
So, like any webpage, you can send data to the webpage using &variablename=value. Fairly simple process, but adding properties to this particular script could make for a long url. I have tried to make this accessable to everyone, so I'll explain the properties.
image: Sets the image using a URL. Does not support variable urls, so sites like Dropbox won't work. Personal websites and imgur will work though as long as you use the direct link to the .png or .gif.
width: This sets the mask image's width.
height: This sets the mask image's height.
camX: Set's the webcam's x. (defaults to 0 if not set)
camY: Set's the webcam's y. (defaults to 0 if not set)
camWidth: Set's the width of the webcam. (Defaults to image width if not set. The way html5 captures webcam feeds, changing the height doesn't change the aspect of the webcam feed, and instead scales the whole image. To keep things acting in an expected manner, only the camWidth can be set, not the camHeight.)