If no image is selected in Keynote, and the slide contains a single image, it will be the recipeient of the import.
If the current slide contains multiple images, the selected image will be the recipient of the import.
If the slide has no images, the Finder item will be imported into the slide and automatically sized.
NOTE: Imported image files will include any embedded description metadata. Also, movie files are compatible for import and will be replace the contents of a target Keynote image frame.