Child theme – overriding add_image_size by a child theme [duplicate]

I am trying to get my child theme to work and to overide one line of code in the main theme functions.php but I can not getting to work since I am totally green in php.

In my main theme functions.php I have:

add_image_size('portfolio-three', 300, 214, true);

When I change it directly, change the image size it works like a charm and resizes images in my portfolio page. But I want to include it in child theme. The code of the functions.php in child theme is like this:

        <?php
    function mytheme_child_scripts() {
        if ( ! is_admin() && ! in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) ) ) {
            $theme_info = wp_get_theme();
            wp_enqueue_style( 'mytheme-child-stylesheet', get_template_directory_uri() . '/style.css', array(), $theme_info->get( 'Version' ) );
        }
    }
add_action('wp_enqueue_scripts', 'mytheme_child_scripts');
add_image_size('portfolio-three', 388, 214, true);
?>

And it does not work, nothing happens, no error and also no image resize.
I do not know what to do here. Could anyone please help me with implementing it properly ?

Best regards!
Jacek

I would like to thank very much cybmeta for pointing me tho the other thread where I could find a solution to my problem. All works perfectly now and I am glad I received help so fast.

I have modified my child functions.php like below:

<?php
function mytheme_scripts() {
    if ( ! is_admin() && ! in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) ) ) {
        $theme_info = wp_get_theme();
        wp_enqueue_style( 'mytheme-child-stylesheet', get_template_directory_uri() . '/style.css', array(), $theme_info->get( 'Version' ) );
    }
}
add_action('wp_enqueue_scripts', 'mytheme_child_scripts');
function child_theme_setup() {
    add_image_size('portfolio-three', 359, 214, true);
}
add_action( 'after_setup_theme', 'child_theme_setup', 11 );

It now works like a charm.
Thank you mr cybmeta and best regards to you and all the community.