Gravatar's site says the default image you specify "MUST be publicly available (e.g. cannot be on an intranet, on a local development machine, behind HTTP Auth or some other firewall etc). Default images are passed through a security scan to avoid malicious content."
The url you are using for your default image is hosted in your development environment on localhost, so that will fail. To make it work you need to host the default image somewhere public and use that public URL for the default.