After doing the test,( please do not ask me if I broke the test site, because I did), I went to my site.

I use WP Fan Zone Theme. Go to Dashboard, Appearance, Editor, function.php

and add the following code :

function replace_content($text)

{

$alt = get_the_author_meta( 'display_name' );

$text = str_replace('alt=\'\'', 'alt=\'Avatar for '.$alt.'\' title=\'Gravatar for '.$alt.'\'',$text);

return $text;

}

add_filter('get_avatar','replace_content');

Notice I added this code after the writing in function.php (look for this)

add_theme_support( 'html5', array(

'comment-form', 'comment-list', 'gallery', 'caption',

) );

Press Save after you add the code!