Filtering the default avatar list

I’m using the avatar_defaults filter hook to filter the default avatar list. For example:

function my_avatar_defaults( $avatar_defaults ) {
    $avatar_defaults['http://example.com/foo.png'] = __( 'Foo' );
    return $avatar_defaults;
}
add_filter( 'avatar_defaults', 'my_avatar_defaults' );

This works because if I visit Settings > Discussion and then scroll down, I can see Foo has been added as a default avatar option. See the following screenshot for an example:

The problem is the src attribute of the image displayed next to Foo. It seems to be making a call to Gravatar. Here’s an example of the source code I’m getting for the image next to Foo:

<img src="https://0.gravatar.com/avatar/efaeb0e0be9922051a1c4ccce766a141?s=32&d=http%3A%2F%2Fexample.com%2Ffoo.png%3Fs%3D32&r=G&forcedefault=1" />

How can I ensure the src attribute points to my image URL instead of the Gravatar URL (which seems to have my image’s URL inside it)?

You are doing it correctly.

The call to gravatar.com passes the location of your custom image so that WP can load it. Take a look at the query string in the src, you’ll see your image location. That’s how it works.