If you haven't changed the default settings for dashboards in system settings try this
<img src="/modules/dashboards/web/public/<your img file>" />
The best place to store your image files as well as all the other files you are using in your dashboard is in the overrides dir. This way when you upgrade Mango you don't have to worry about your files being deleted.
<Mango Root>/overrides/web/modules/dashboards/web/public