Fix broken image preview

Author: leemunroe

README.md

@@ -7,7 +7,7 @@
 
 [OneSignal](https://onesignal.com/) is a free push notification service for mobile apps. This SDK makes it easy to integrate your Unity app with OneSignal.
 
-![alt text](https://onesignal.com/images/android_and_ios_notification_image.gif)
+<p align="center"><img src="https://app.onesignal.com/images/android_and_ios_notification_image.gif" width="500" alt="Unity Notification"></p>
 
 #### Installation and Setup
 See the [Setup Guide](https://documentation.onesignal.com/docs/unity-sdk-setup) for installation and setup instructions.