You would need to get permission from the person who took the photo or who has ownership of it.
If you email and ask and they respond that it is okay, be sure to print out that respone and save it!

You should defiantly ask permission from the photographer. An easier alternative might be to do a search for "stock wedding cake pictures" these are free pictures for anyone to use for any thing.

You could always make one yourself and take a picture of it. Just put your card in the picture so it's clear who made the cake. I know a lot of girls use cake photos in their bios on the Knot and other wedding websites without asking but you want to use it for a business, I presume a bakery or wedding related business. So you want to get permission. There's a good chance they won't mind if you are out of the area where they do business or if you identify the baker of the cake/designer etc.