You open the water hole photo, but DON'T open the tree photo. With the water hole photo open, you'll go to File>Place, and select the tree photo. After placing it, you reduce its layer opacity to about 50% so you can see where you're putting it, then Ctrl T to resize and move it to exactly where you want it. Then increase the layer opacity to 100% again. Then add the mask, and mask it in.