GoToFullScreen makes a video (running on the bottom-left corner) go to full-screen, but I don't know if it would do that to an image.
If you are in an MCML application, you could always just have an image the size of your screen (eg. using a FormLayout with the anchors at Parent,0 for top and left and Parent,1 for bottom and right) that you change the visibility of at runtime.
In Yougle (a program that shows images at fullscreen) I actually have a seperate MCML page with just this Graphic object. I can then transition into the (and it's black background) using a Show animation. I navigate to that page based on a button ( a smaller thumbnail of the image ) being pressed.