I think you have to use iTunes to designate which of Aperture's images you wish to share with your ATV.
Under "File" in iTunes you'll find "Home Sharing" and "Choose which images to share with ATV". I find it takes some time for 1) the list of Aperture images for sharing to show up and 2) once you have made your choice, checked the appropriate boxes, and clicked "Apply" for the new picks to become active.
My problem has been updating my sharing list when I add new images to Aperture. It seems to require a combination of re-running Aperture, turning home sharing "on" and "off", and changing which images get shared.
Whatever, it's pain in the butt!