Hey , Ancy ,Maybe I got your problem . First ,Give you some Shortcuts Keys for capture screenshot on Mac
Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
Command-Shift-4, then select an area: Take a screenshot of an area and save it as a file on the desktop
And you just can try the Screenshot Capture for Mac (macscreenshots.com/screenshotcapture_mac.html),which can capture frame region, freehand region, one or all windows and full screen on Mac .
Hope it will be helpful to your project .