There is a very simple 'Automator' action for taking screenshots which allows for timed screenshots.
Run 'Automator' on your Mac. Select 'Application' type and click choose. Then select 'Utilities' from the 'Library' column' and then double click on 'Take Screenshot' from the second column. Then configure the action as required in the right panel.
You can then save the application so you can run it from the finder whenever you need it.