Pictures speak a thousand words is a saying and moniker that has been around pretty much since the early years of the introduction of the camera. While a book has words and its only as good as its weight, a picture stands the test of time. It gives a visual intrepretation of what object or scene that is being photographed.
Those thousand words could pertain to the object being photographed right on up to what was going on in the world at the time. A picture is able to conjur up so many thoughts and memories that being able to put it to words would be too difficult to do. It would certainly take a thousand words or even more to put it just exactly how you would want it.
In all reality it would be most difficult to describe a picture. A picture speaks a thousand words because it is a visual interpretation and it would take a very, very large amount of words to describe exactly what the picture is trying to say.
When you are explaining something to a person, you are trying to create a virtual image in his mind. Depending on your skills of explanation, the image can be along the lines of what you think. However, it can never be the same as something that you have in your mind.
A picture on the other hand shows what you need to show rather than explain. This means that it will be portrayed as it is to the every person without a doubt. This is why it is said that pictures speak louder than words. Clearly, a picture is able to deliver the idea better.