If no image renderer available/installed when adding a new  alert notification 
channel will set Include image to false per default, otherwise true as before.
If no image renderer available/installed when adding/edit alert notification 
channel will show an informational message that you need to install the 
Grafana Image Renderer plugin for Grafana to be able to render an image. 

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>