Green’s Dictionary of Slang

capper n.2

[SE screen capture]

one who hunts down images of (usu. young) women, half-dressed, naked or performing sexual acts for a screen camera; that image is then captured and circulated on the Internet.

Urban Dict. 27 Feb. [Internet] Capper A person who spends every hour of every day on the internet trying to record random girls on webcam and making friends with other geeky/lonely pervs.
[US] 21 Oct. [Internet] That is what so-called ‘cappers’ do [...] They screen-capture live webcam chats — which can involve anywhere from two to several hundred people — while pressuring girls and young women to strip down.