Not sure how to do screenshots on the Rigol yet.
Put a 4 or 8 GB capacity USB stick in the port at lower left next to the on-off switch. Larger drives may work too, or not. Wait a moment for the scope to recognize the stick. Then press the green button with the "Printer" symbol on it, just below the green "Help" button, at the upper right of the front panel. The scope will save the screenshot to the USB drive as a .png image file and give it a sequence number. Unfortunately since the scope does not have a realtime clock, the file save date/time will not be correct. Because of this it may help you to make a note of the sequence number and what the screen is showing, for your records. Or shift the file to your computer and rename it appropriately.
Saving a screen takes a variable length of time. Sometimes it will be very quick and other times it takes a while. More complex setups may save more quickly if you stop the scope with the Run/Stop button before making the screenshot.
The scope certainly does have the resolution to show a 50 ms delay between two signals on different channels! If, that is, it is set up properly and the probing is done properly. So if possible please provide a schematic or block diagram with probe points, and a screenshot or two of what you are getting on the scope.