I've created my own service like this. The goal was to provide an archive of images for clients, showing their website's content over time and 'proof' of this or that. The setup involved a system to create VNC/xvfb screens, which ran a real browser and created a snapshot further processed with imagemagick. This required a bit of a larger VPS than I would have needed otherwise. There was some overhead and complexity. I researched the systems available at the time, and most of them were either too expensive or inadequate (i.e. only provided thumbnails, and we needed full size snaps).