This The Wiggles icing image is approximately 16.5cm.  Completely edible.

To place on buttercream:

Cover your cake with buttercream and smooth.  Carefully remove image from backing (place in freezer for 5 mins if needed to help with removal), then place on top and press down.

To place on fondant:

Cover your cake with fondant.  Using a slightly damp brush, dampen the area the image will sit on, or alternatively, use piping gel.  Remove image from backing as above and press and smooth.

Photo is indicative only - printing with edible inks is duller than regular photos.