Definitely infuriating!!! Being upset is totally reasonable. I agree that being asked would have been a lovely compliment, but it still would have been your decision to agree or not. Not asking is downright thievery! So what can be done about it? I would write to the thief directly with your proof (the links that you included here) and ask either that she/ he remove the item being sold from her / his site or at least give you credit AND PAYMENT! She/ He is not just presenting the photo on her/ his site as her/ his own, but is charging money for that exact cake to be made and sold!
The Garden Baker
