Using the Digital Watchdog software (PC or mobile), choose the camera in which the event happened. 

Choose the camera's 'live' feed. (double click or tap)

Locate the timeline graph below the camera feed, select the timeline (pinch to zoom/scroll wheel to zoom)

Default

Zoomed in

Locate the quick tools menu (upper right of live feed screen)

Select the screenshot tool 

On PC: After tapping or clicking on the screenshot button, you will be met with a window to determine where to save the still photo. 

On mobile: Still photos will save to 'Photos' or Gallery (iOS & Android respectively)