Load both pics into PS
From the layers menu, drag the thumbnail of the pic (pic 1) into the second pic (The one of them holding the phone).
Close original girls only pic (pic 1) to dump it from screen
Shrink the image a little (Press Ctrl+Z to get zoom tool, then hold down Alt key and left click in the pic to zoom out)
Zoom out till you can see the image in it's entirety.
Ensure the layer of the "girls only" is active, and then press Ctrl+T (T for Transform)
You'll see the image now has an outline and little blocks at each corner and also at the midpoints top,bottom and sides.
Hold down the shift key and left click in one of the corner boxes and drag it towards the middle of the image. This causes the image to resize proportionally.
Once approximately the right size, release the box, and then release the shift key.
Now drag the centre of the image over to the mobile phone screen.
Not quite the right size? Repeat above steps regarding shift key and corner drag.
Now to align it - move the mouse cursor to one of the corner boxes, but move the cursor just outside the image. You will see your cursor change to a double ended curved arrow. When you see this, left click and drag to rotate the image.
Once happy with the size and orientation of the image, simply double click in the middle of the adjusted image layer to commit the changes.
One last thing, reduce the opacity of the layer or darken it a little, so it doesn't look fake. It needs to look similar to the screen on the untouched image of the mobile screen.
