Hello! To "do something justice" means to treat, or represent someone or something with due fairness or appreciation. So in the context of your sentence, it means that the original place is way more beautiful than the picture that is depicting it. This is because it is impossible to replicate the true three-dimensional view of something beautiful on a small piece of cardboard. I hope that helps!

The sentence means the picture on the postcard doesn't display the real beauty of the place it is representing. For example let's say a beautiful looking beach but the photo is very poor, hence it does not look as good. Hope this helps :)