Set default avatar network-wide

The avatar_default is the better option.

add_filter('pre_option_avatar_default', 'se72578_avatar_default');
function se72578_avatar_default( $option ){
   return 'http://example.com/your/default/here.jpg';
}

Simple. Stick that in a mu-plugin and you’re good to go.

Edit: If you want to still allow site owners to change the default avatar, use the default_option_avatar_default hook instead.