Hello,
You can use com.webos.service.capture service to make screenshots:
luna-send -n 1 -f 'luna://com.webos.service.capture/executeOneShot' '{"path":"/tmp/usb/sda/sda1/NAS/shares/capture.png", "method":"DISPLAY", "format":"PNG"}'
... then copy it to /tmp/usb/..... or other stuf.