Both are right. Defamation is a general term that encompasses both libel and slander. Libel is written, slander is spoken. When in doubt, just say "defamation."

> a screenshot cannot possibly be used as evidence

Generally speaking, I see no reason why this should be the case. And screenshots are routinely used as evidence in U.S. courts. Of course, if the opposing side challenges the accuracy of the screenshot, then you'll need to give more evidence (testimony, probably) about how it was produced. But that doesn't mean that screenshots are per se unreliable.