Do humans only dream of things they have actually witnessed/seen? Is it necessary for humans to have dreams that only contain things they have witnessed/seen in their consciousness?
People often report dreaming about  things that they have not experienced in the real world (e.g., monsters, flying, falling off a building). 

With our own imagination while awake most people can visualise many things that they have never actually seen. It seems likely that dreams would be at least as flexible as our own imagination. 

Videos, stories, images, and other media might inform this imagination. Likewise, in general, daily thoughts, emotions, and experiences are often weaved into dreams. 

This is just a basic common sense answer. It takes for granted an information processing model of cognition that denies the existence of true psychic power in dreams. That said, a weak form of what might be labelled "psychic" or "premonitions" in dreams is still possible in the form of insight that was only first achieved in a dream. More deeply, it would be interesting to see another answer that compared the kinds of creative imagination exhibited in dreams relative to creative imagination while awake.