Many camera owners believe dust spots on pictures are caused by dust specs they see inside their lens but this is a misconception. Individual specs inside a lens will not show in pictures as defined spots. 

If a dust spot inside a lens is large enough, it could cause a distortion in the image but is not likely to show as a dark area. However, If a large enough spot is on the rear element of a lens it may cause a large, shadowy looking spot in the image.