change alt attritube on JNews featured gallery

add_filter( 'wp_get_attachment_image_attributes', 'change_alt_attribute', 99, 3 );

function change_alt_attribute( $attr, $attachment, $size ) {
    if ( 'jnews-750x536' === $size ) {
        $attr['alt'] = $attachment->post_excerpt;
    }
    return $attr;
}