Add logo above menu i twenty twelve

Do you have a file named ‘header.php’ in your Child Theme directory?

A Child Theme is just a folder in the /wp-content/themes/ directory, separate to your theme. You then write some CSS to @import the styles from your actual theme over. This then gives you an environment (in the Child Theme) to modify the theme, without modifying the theme’s files.
http://codex.wordpress.org/Child_Themes#Example_of_a_basic_Child_Theme