There are multiple ways to add a picture in Keynote:
Drag an image from your computer or a webpage to a media placeholder or anywhere else on the slide or the workspace around it.
Click in the bottom-right corner of a media placeholder, then select an image from a photo library.
Click in the toolbar, choose Photos, then drag an image from a photo library to the slide or to a media placeholder.
Click to select the image you want to replace, then in the Format sidebar, click the Image tab. Click Replace, then choose an image. The new image retains the dimensions of the original.
For more instructions on adding images in Keynote, head HERE.