Images of well-known characters, movies, and things of that nature usually fall under what's referred to as fair use copyright, and you're generally allowed to use and share common images from shows and movies as long as you properly attribute the source. The same goes for talking about well known characters, or writing things like fan fiction—as long as you're not claiming the characters or images to be your own, you can discuss fictional characters and plots in your stories, and even use them as inspiration.
You can read more about fair use copyright here.