In Dos command to take screenshot, you can use (see https://developers.google.com/web/updates/2017/04/headless-chrome):
chrome --screenshot="C:\test\test.png" "http://google.com" --headless --disable-gpu
In Rebol, I can make it work only using powershell -command:
call {powershell -command "chrome --screenshot='C:\\test\\test.png' 'http://google.com' --headless --disable-gpu"}
In red I cannot make it work at all, did I miss something ?