How to mask images in Keynote into circles?
Yes.

This works for any shape. Here's how:


Draw a shape.
Resize it to the desired size.
Position the shape over the image.
Select both by shift-clicking them.
Choose Format → Image → Mask with Shape from the menu (these menu items are from Keynote 6.5.2, might be different for your version)
Mask the image as you'd usually do (by dragging, resizing).
Click Done.