How to replace “Password Protected” text with icon in Admin

Try this (don’t forget to replace icon URL):

add_filter( 'display_post_states', 'password_protected_icon' );

function password_protected_icon( $post_states ) {

    $text = __('Password protected');
    $pos = array_search( $text, $post_states);
    if( false !== $pos )
        $post_states[$pos] = '<img src="http://i.stack.imgur.com/aIDa6.png" title="'.htmlspecialchars($text).'"/>';

    return $post_states;
}