Hi Shelley! What a magical time your little princess is going to have at the
Bibbidi Bobbidi Boutique! I think pictures of the current princess gown choices would be a great addition to the Disney website, but unfortunately they aren't there at this time. Unofficial websites like disboards.com have message board threads with extensive photos of the costumes at the Boutique so you could give your daughter an idea of her choices by looking at photos from recent visitors on the Theme Parks Attractions and Strategies board there.
Because merchandise is always subject to change, it may be better to have your daughter decide in advance the princess that she'd like to be. I'd hate for her to have her heart set on a particular costume and then be upset if the one at the Boutique is not exactly like the one she'd chosen. Costume choices generally include Ariel, Snow White, Aurora, Belle, Jasmine, Cinderella, Tinker Bell and Tiana. Another option would be to purchase her dress at www.disneystore.com prior to your trip so she'll have lots of time to browse the selections without taking time away from your family's park visit.
Hope your little princess finds the perfect gown! Come visit us again at the Moms Panel if you have more questions! We'd love to help!