Many animal shelters have pictures of the dogs and other pets that are up for adoption available on their websites. Often times, pictures of shelter pets are also on display at local pet food stores and veterinarian offices.
Lucky Dog Animal Rescue is an excellent resource because it has a comprehensive search engine to find the perfect pet. Each dog not only has a photo, it has a profile that includes location, approximate age/weight, adoption fees and background information when available. Another option is the PetSmart website that gives a list with links to animal shelters in all fifty states, most of which also have pictures of available dogs and other animals.