You could display an image on a dashboard using the Markdown visualization, and make the image tag in Markdown like:
You can also drop an images in the src/ui/public/images folder to have the image hosted by the Kibana web server. If you do this, you will need to run the Kibana optimizer to be able to access the image from a browser. You can do this by installing some plugin, for example Sense:
./bin/kibana plugin --install elastic/sense
After you restart Kibana, you should be able to find the image at: http://your-domain:5601/bundles/src/ui/public/images/your-image.png