There are different ways by which a Apple Keynote user can add an image to Presentation. The quickest option is to drag and drop the image from another application or from the Mac desktop.
Click the Insert menu and select Choose from the list of available menus.
Then pick the required image and click the Insert button.
Apple Keynote users can also add images using the Media Browser option available at the top right hand side.
Launch the Media Browser and navigate to the Photos section. This would allow you choose the available photos from the iPhoto App.