Now that you've added a picture, it needs to be faded or washed out so it won't compete with any text or other content on your slide.
To change the brightness or contrast of a picture, you use the options on the Picture pane in the Format Background dialog box. Click Recolor to open the gallery, where you can apply a stylized effect (such as washout, grayscale, or sepia tone) to a picture, and then, under Color Modes, click Washout.
You're almost done. All you need to do now is save your theme so you can reuse it. And you'll see how to do that in the next section.