You can easily replace an image on a layout, or swap images on the same layout, or on different layouts.
To switch images on the same layout:
Drag and drop one image over the other. They will change places.
To switch an image with another on a different layout:
Click one of the images you want to switch and drag it to the side of the screen, which will allow you to scroll through the layouts. When you get to the layout you want, drag the image you're moving over the image you want to swap with, and drop. The images will change places.
To swap an image for one not yet added to the album:
Simply drag the image from the image panel onto the aperture you wish to add it to. The image already in the aperture will be replaced.