How can I combine 2 images using the 3rd alpha mask image

Question

I have 2 images, foreground and background. I want to combine 2 using an alpha mask image, so black = use a background pixel, white = use a foreground pixel, and gray is a mixed alpha mix of 2.

The source and background images are fixed, and the mask image is created on the fly, and then moved as needed to create the animation.

Is there any efficient way to do this using the standard library? I can use LockBits and a loop, but I would like something faster if such a thing exists.