In reality, using the alphabet provides more variety of subjects than numbers. For example (if using numbers), I could post photos of sled dogs starting with one dog, then dog teams starting with 2 dogs and ending with 16 dogs (usually the largest dog team). It means that eventually, if using numbers, I would have to post an entire kennel full of dogs. If using people in number, then I would have to post a photo of a crowd of people, and we already know how difficult it is to count a great number of people in a crowd.