adding new custom resolution to set under thumbinals “edit”

Hi there, I’m wondering how to add custom resolution to the list that is displayed after clicking “edit” on each thumb.
So far I’ve try to add:
//
add_theme_support( ‘post-thumbnails’ );
add_image_size( ‘custom-thumb’, 480, 480, true );
//
to my functions.php, but it appears only between “Image size” and “Want to add more images sizes? Read a simple tutorial.” in dropdown-list of “add images” section. Even after clean upload with 480×480 selected there is no effect.
I’ll be grateful of any advice.