Creating image from two other images using javascript

Question

I am having a slight problem, I need to be able to create image file from two other images, and I have to able to place other image over the other one. Example:

Blue_box.png:

Yellow_box.png:

And I have to let javascript create a new image file that would look something like this:

Final_box.png:

And I have to be able to place the yellow box using cords