Custom Gravatar not working

I think the problem is simply that your default avatar isn’t publicly available.

Try for example

http://upload.wikimedia.org/wikipedia/en/b/b0/Avatar-Teaser-Poster.jpg

instead of

http://loalhost:888/wordpress/wp-content/themes/blog-theme/img/icons/avatar.jpg

Notice that the default gravatar is served via http{|s}://i{0,1,2}.wp.com/...

From the docs:

When you include a default image, Gravatar will automatically serve up that
image if there is no image associated with the requested email hash. There are a > few conditions which must be met for default image URL:

• 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.
• MUST be accessible via HTTP or HTTPS on the standard ports, 80 and 443,
  respectively.
• MUST have a recognizable image extension (jpg, jpeg, gif, png)
• MUST NOT include a querystring (if it does, it will be ignored)