Photos ordered through iPhoto must match the aspect ratio for given print sizes or they may be cropped automatically.If you take a digital photo you'll need to constrain and/or crop your 4:3 aspect ratio image, when printing a 4x6, 5x7, 8x10, 16x20 or 20x30 print.
Or you can do as I do and just let Apple do it for you.
Works for me.