feat(alerting): add image for howto popup

closes #5784

public/app/features/alerting/partials/alert_howto.html

@@ -12,6 +12,9 @@
 
 
 	<div class="modal-content">
+		<div class="text-center">
+			<img src="public/img/alert_howto_new.png"></img>
+		</div>
 		<p class="p-a-2 text-center offset-lg-1 col-lg-10">
 			Alerts are added and configured in the <strong>Alert Tab</strong> of any dashboard
 			graph panel, letting you build and visualize an alert using existing queries.