This is a simple function that should work for you:
function add_custom_default_avatar($avatar_defaults){
$custom_avatar = 'https://{imageURL}';
$avatar_defaults[$custom_avatar] = 'Custom Avatar';
return $avatar_defaults;
}
add_filter('avatar_defaults', 'add_custom_default_avatar');
You would paste this into the functions.php file in your theme folder. You can paste it at the very bottom of the file, or if there is a ?> at the bottom, paste the code just right before that.
Replace the {imageURL} with the URL of the image you want to use. The avatar should appear in your Discussions settings now, and you can switch the Default Avatar to it.