Screenshot software

I’m looking for a great screenshot program that runs on Windows, that can be programmed to run in the background and automatically (without any user intervention) take a screenshot of a website page and save the image as a jpg or png. In particular, it needs to be able to take a screenshot of any responsive website page and perhaps include a prescribed delay in order to wait for the full page to load.

I’m wondering what Webpagetest uses as it’s very successful in always creating a screenshot.

If not Webpagetest’s solution, I’d love a recommendation or two as I’ve not been successful finding one that works well.

Thanks! Rick