ApiFlash is the most robust and cost efficient website screenshot API out there. You just have to sign up and make a GET request to the main API endpoint with a few query string parameters to get a screenshot. The API does scale massively as it is built on the Amazon Web Services platform and especially on the Lambda serverless service. It can make millions of screenshots per day to satisfy even the biggest clients needs. The variety of parameters are flexible enough so that you can tweak and get the exact screenshots you need.