This all depends though on what the agreement was. There are some people who think they are great bakers, and "offer" to bake THE birthday cake as a present, but truly, they are not good at it at all. Unless you've baked quite a few cakes, for example, there's no way you can create a good "topsy turvy cake" for example, but there are people who think they can. Many people can't say no (I would politely decline), but really, for a child, a birthday cake is something special - not just something to be eaten, but something to look at. Not many mums would like to serve a crumbling mess.
I bake cakes from time to time. Normally, they tend to be very good. But I would never offer to bake a child's birthday cake that isn't my own. At least, if I was baking it for my child, and something went horribly wrong, I could go out and buy one as a last resort.
The OP's SiL is not even given the opportunity to see whether the cake went badly or not!! She probably just wants to see whether she has to do a mad dash to the shop, and combined with birthday party stress, it is perfectly reasonable to want to see a picture.
This thread takes that saying "do not look a gifted horse in the mouth" quite literally. Anyone who knows anything about horses will tell you otherwise...
Just looking at it from another perspective... 