I have a bunch of photos that I need on a single slide, and I'd like them all to be shown as circles.
Can I do that out-of-the-box with Keynote?
This works for any shape. Here's how:
Actually, it is unnecessary to create a shape in order to mask the image.
Now drag the mask as desired.
Set this image as a Media Placeholder by selecting the image then, Format > Advanced > Define as Media Placeholder
Copy the image as many times as desired on the slide and then drag new images to them. The mask will be applied to the new image as well.