markf1 on "Change Thumnail Size in Twenty Twelve Child Theme"

I am trying to change the thumbnail size in a child theme of Twenty Twelve

I added a funtions.php file to the child theme with this code in it:

<?php
if ( function_exists( 'set_post_thumbnail_size' ) ) {
    set_post_thumbnail_size( 100, 100, true );
}

When I select a featured image for a post it shows up as a thumbnail but in the Twenty Twelve default size of 624px wide. I would like the thumbnail to be much smaller - 100, 100.

Clearly I'm not putting the correct code into the child theme functions.php file. Any help with this will be very much appreciated.