Choose your theme
For this next step I'll open a blank version of the theme I want to add this layout to:

In this example I have chosen the theme that my cutout matches
Dupe a Master Slide
Once you chose the Theme you want, slide down the bar that covers the master slides, so you can get at them:
Next, pick a master slide that ALREADY HAS a photo cutout on it to use as a starting point for your new slide layout and then COPY it:
Then Choose paste from the Edit menu and rename the slide to whatever you want:
Remove the old Cutout
Now, click on the Master you just renamed and then click on the background, in an area where there are NO text boxes. This SHOULD select the photo cutout that's already on the slide. Choose Unlock from the Arrange menu and then delete the unlocked photo cutout:
This will leave you with a blank slide that may have text, but has no photo cutout on it.
Drop in the new Cutout
The next step is to drag the cutout you made earlier onto this slide:

I just dragged this file from the Finder onto my Master Slide
Once you've got the cutout on your slide, you need to make sure it lines up so it covers the entire slide. The easiest way to do this is with the Measurements Inspector:
Send it to the back
At this point, you should see something like this, with the cutout covering some of your text:
Click on the cutout to select it and choose Send to Back from the Arrange menu: