Could you please give us more information about your issue? Do you have any screenshot of it? I think I am not able to reproduce your issue.
As you can see in the image below, I created a new topic and the default avatar is properly shown in both the preview and when I click the icon to get a bit more information about that user. I launched a fresh new Discourse image from the AMI ID you posted. I didn't install any plugin nor did any other modification to the image.
Also, I navigated to the IP address you posted but it is giving timeout errors. Did you stop the server?