It's two parts: An App Engine/Python for the UI and user registration, and a Linux server for image capturing (Python, Javascript, RabbitMQ, ...etc.). At first we used popular libraries like Phantomjs, but eventually had to write our own to scale better.
I'm experimenting right now with moving the image capturing code to an App Engine managed VM to simplify things and make it scale easier.
Could you please elaborate on what your own implementation turned out to be?
Which library are you using for image capture? Been using webkit2png ( https://github.com/AdamN/python-webkit2png/ ) for a personal project, but I'm interested to hear your experiences.
Could you mention why you stopped using PhantomJS? Also, how are you supporting flash?