To do this duplicate the background. Click on the vector mask button. Its at the bottom by duplicate layer. Its a circle with a dark part around it. Take a brush and change the color to black. Then just brush over the portions you want to go away.
rough version. but your pic is fairly easy to do, cause the ground is all the same.
^note, that background is transparent. you just cant tell, cause this website has a black background. save it, and put it in photoshop, you should be able to put the background of whatever you want behind it. With Layers.