evil resha - What's the problem with looking at a wicked person? And who does it apply to?

Why does Jewish tradition (the Talmud ) say that it is forbidden to look at the face of a wicked person? Does it apply to looking at a picture of a wicked person? And how wicked does a person have to be told be included in that category ? Are we talking about Hitler wicked or even a neighbor who speaks loshon harah or tasks during davening?