breathelifein on "Creating a new image size and having it as an option on the Media Library"

I'd like to create extra, custom image sizes, and then be able to chose from them when inserting an image to a post using the Media Library.
I tried following this post from the Codex [http://codex.wordpress.org/Function_Reference/add_image_size] and some others I found when googling, but I can't seem to be able to make it work.

I added to my functions.php:

if(function_exists('add_theme_support'))
add_theme_support('post-thumbnails');
add_image_size('thumb_small', 100, 127, true );
add_image_size('thumb_medium', 80, 102, true );

Then

add_filter( 'image_size_names_choose', 'my_custom_sizes' );
function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'thumb_small' => __( 'Miniatura P' ),
        'thumb_medium' => __( 'Minuatura M' ),
    ) );
}

But still, these size options don't show on my Media Library.
What am I doing wrong? How can I get it to work?
Thanks.