To add the rainbow colors to the image, all we need to do is drag out a gradient. I want my rainbow to appear as if it’s touching the ground behind the group of trees on the left of my photo and then arching high above the larger tree on the right. To do that, I’ll position my mouse almost at the edge of the photo near the bottom right corner, in line horizontally with the bottom of the trees on the left. I’ll click on that spot to set the starting point for my gradient, and with my mouse button still held down, I’ll drag all the way across the image to a point just beyond the left of the trees. Holding my Shift key down as I drag will limit the direction I can drag in, making it easy to drag horizontally: