Do a Google images search:
1. Go to: http://images.google.com/
2. Click on the little camera icon in the search bar.
3. Copy and paste the link to the photo, or...
4. Save the photo to your hard drive and then upload it to Google.
It will show you websites where that photo appears.
You can also type in “wedding dress” into http://images.google.com/ and see a bunch of them, and follow the links to the retailers.