No No No No No No No do NOT refund ANY money! She has no right to demand a refund for something like this. You are not responsible for what happens to a cake after it's delivered NOR how she think it photographs! I suppose if she looked fat in her pictures she'd demand her money back on her dress?
Please don't get suckered into something like this. I'm assuming you have a contract...and in that contract it should state the very fact that once the cake is delivered and set up, you are not responsible for what happens to it.
Anyway, I would reply to her something like this (in an e-mail....always helps to have a paper trail):
Dear Bridezilla (well you edit that),
After receiving a rave review from you on your wedding cake several months ago, needless to say I was very surprised and saddened to hear that you are now unhappy and requesting a refund. I'm sorry you don't think the cake photographed well, but that is really not my responsibility. The cake was designed to your specifications, delivered, set up --AND I assume eaten. Any damage that occurs after delivery is out of my hands. The money you paid me has long ago been spent on bills--including the cost of making your cake. I regret that you are unhappy and while I will not be issuing a refund, because I appreciate your business I hope that you allow me to make your 1st anniversary top tier at no charge.
Sincerely...yada yada yada.
By offering something like her top tier, at least you're being gracious. Which is more than she is! Above all, do NOT cave! I wouldn't go back to a restaurant after several months and tell them my steak was tough and I want my money back! Please! That cake is GORGEOUS! And any photographer worth his salt can work wonders post production if there are any flaws. You might tell Bridezilla that too.