With the Move tool, click-and-drag the earth photo selection onto the other photo.

Click-and-drag a corner of the earth layer. If there's no transform controls on the corners, make sure Show Transform Controls is checked in the Control palette. Lower the opacity of the earth layer in the Layers palette to be able to make it the correct size.

The earth needs to be a little bit bigger than the red ball here, since the ball isn't completely round.
