Your code actually works, it is just that you can’t do what you want.
The way gravatar works is by serving the image if it has one and redirecting to the supplied default if it doesn’t, so even if the default is on your server you still can’t avoid the redirect.
And gravatar really makes your site load faster
- makes your server less loaded
- enables the browser to make one more simultaneous request to your site to fetch something else (browsers are limited to 6 simultaneous request per domain)
- gravatar’s mystery man is probably at a CDN and served from a place much “closer” to your user than your server
- gravatar’s mystery man is probably already in the browser cache and no request is done to fetch it, your mystery man is unlikely to be there.