I copied an image into the clipboard: but there is no paste option within Keynote. So.. how to do this basic operation?
The general answer is
"just be sure to paste into an unoccupied area of the Keynote slide"
My issue was that I was trying to paste into an area already occupied by a textbox. It may not be obvious due to a bunch of blank lines that make the box extend longer than immediately apparent.
Be sure you are on the slide by clicking into it. The universal keyboard Command + V should pass your image. If that doesn't work then be sure you actually have something copied to the clipboard. Alternatively you can easily drag and drop an image onto a slide.