In QlikView some of the visualization were actually images rendered on the Server. In Qlik Sense visualizations are drawn on the client and mostly in Canvas so no images are available.
So to be able to output visualizations as a image we usually render the vis on the server in a headless browser and screenshot the output. That is what the Printing service is doing in-side of the Client.
The printing service is currently not exposing it's API but that is something we are currently working on.