I stumbled onto another method that would work ffmpeg.
To save a picture from the RTSP stream this works but you need to disable authentication for ONVIF for it to work. Normally you can not disable authentication for Dahua cameras but I found in my Amcrest camera it is possible to leave it off for ONVIF whilst the rest stays protected via digest.
In rules you can use the execute method to run this command.
ffmpeg -t 1 -i 'rtsp://192.168.1.50:554/cam/realmonitor?channel=1&subtype=0&unicast=true&proto=Onvif' -vframes 1 /etc/openhab2/html/camerapicture.jpg
More info here…