Use "overlay" for set image overlaying the chart - not possible (so far/yet) save the chart as a PNG image with the overlayed image. See: https://stackoverflow.com/questions/46082246/save-a-google-chart-as-png-image-with-overlays
Sources:
https://developers.google.com/chart/interactive/docs/overlays#javascript
http://jsfiddle.net/asgallant/R8A8P/
http://jsfiddle.net/rathore_gee/Lfjgjzyy/