Expand the Phone layer group then doubleclick on the thumbnail of the "Edit This Layer" layer.
A new document will open. Drag and drop your image onto the canvas then save and close the document.
The phone should now contain your image. Adjust the layer opacity to change the screen brightness.
You can also adjust the opacity of the "Reflections" layer and hide/show the Stylus port.
Select which objects you want to include by hiding or showing the layers inside the "Hand" or "Desk Objects" group.
Select a background by hiding or showing the layers inside the "Backgrounds" group. To use your own background, simply hide all of the layers inside the "Backgrounds" group then place your image below the layer group.
Tip: Remember to hide the stylus port layer if you are showing the stylus or the hand holding the stylus.