Add default ALT to avatar/Gravatar Images

add_filter(

	'pre_get_avatar_data',

	function ( $atts ) {

		if ( empty( $atts['alt'] ) ) {

			if ( have_comments() ) {

				$author = get_comment_author();

			} else {

				$author = get_the_author_meta( 'display_name' );

			}

			$alt = sprintf( 'Avatar for %s', $author );

			$atts['alt'] = $alt;

		}

		return $atts;

	}

);