Apple Keynote 3 - Manual (Page 11)

Preface Welcome to Keynote 11
Free-Form Curves, Shapes, and Masks
Create any shape using Bézier curves. Crop images by using masks of different shapes.
Choose from predrawn shapes or create your own custom shape to use as a mask.
With a mask, you reveal only the part of an image you want to show on a slide,
without altering the original file.
Create a custom mask to
frame part of a photo.
Position the mask over
the image, and then
optionally edit the mask.
Here, it’s rotated and
resized.
The original image is
framed by the mask.