Open the designer and toggle the cameo 'on'.
From there, drag an image from your image bank directly into the cameo window on the sidebar. You can move and scale the cameo image in this window.
Note: do not drag the image onto the album cover mockup
*If you are using pre-made spreads, you will have to upload the single photo that you want to use as your cameo separately, as pre-made spreads will not work as the cameo.