I've shared some TV-side apps lately; many of their screens I photographed directly — intuitive but not pretty. So I looked for a way to screenshot the TV itself.
Found it: connect a keyboard to the TV box (same for a TV), and press Print Screen on the screen you want to capture (some keyboards label it "Print").
To get the image onto your PC: install ES File Explorer on the TV, open the app, find the "access from PC" option, then enter the FTP address in your PC's file manager (TV and PC on the same network). Screenshots are usually in Pictures/Screenshots.