Go To Dashboard → Settings → Media.
Uncheck the following setting.
By default, WordPress organises media files into month and year based folders by default. You don’t want this. So make sure that box is unchecked.
The reason you probably don’t want this is because your photos could end up in scores of separate date-based folders. Some of these will be empty if you do not post regularly. This means, you create longer, pointless file specifications for each of your images.