You take a photo in the store and go to the bakery section. Normally by the bakery section they have the cakes (choc or vanilla) in various sizes and you just pick the one that suits (we opted for two large which were £14 each and take it to where they bake the bread and they'll do it for you while you shop!
I did exactly that too StarryEyedMama. My DDs were older when they got Christened because my OH wanted his kids to get Christened when they were old enough and interested enough to ask for it themselves - don't ask!.
I dressed them up in their christening outfits (bridesmaids dresses really) and took them (carefully) into the field behind my house and took a few photos of them in their dresses with the church they were to be christened in as the background of the photo. It was lovely and made a beautiful cake.
I also made my own fruit cake in the shape of a teddy.