Hi bear3002,
Those are great questions. First off, you do want to put the photo under one of the clear marbles. To do this, you would stick the photo to the clear marble using one of the large stickers that also come with the kit.
As far as the size is concerned, you want the photos to be no larger than about the size of a nickel. This is to ensure that there is a border around the photo for the sticker to seal out any moisture that might seep in. Also, as an extra precaution, you might want to brush a layer of clear nail polish or simiar sealant to further seal out any moisture.
I hope this information has been helpful and if you would like more hints for the Family Photo Stone, visit the 'Family Photo Help' section on our Stepping Stone Kits forum, or feel freee to email us at
marketing@milestonesproducts.com.
Thanks!