How to change WooCommerce thumbnail crop position?

To change existing images sizes (crop option) within your active child theme or theme, you need to use 'after_switch_theme' WordPress hook.

Since WordPress 3.9+ an awesome new feature among many, is the added ability to now control crop position of images uploaded in WordPress.
I don't know if crop potion advanced option is available to woocommerce images sizes, you will have to test it.

The available options for crop position are:

left topleft centerleft bottomright topright centerright bottomcenter topcenter centercenter bottom

So based on this snippet from wooThemes and and (this relative new) crop options of WordPress, you can try this:

function yourtheme_woocommerce_image_dimensions() {    global $pagenow;    if ( ! isset( $_GET['activated'] ) || $pagenow != 'themes.php' ) {        return;    }    $catalog = array(        'width'     => '300',   // px        'height'    => '400',   // px        'crop'      => array( 'center', 'bottom' ) // New crop options to try.    );    /* $single = array(        'width'     => '600',   // px        'height'    => '600',   // px        'crop'      => 1        // true    );    $thumbnail = array(        'width'     => '120',   // px        'height'    => '120',   // px        'crop'      => 0        // false    ); */    // Image sizes    update_option( 'shop_catalog_image_size', $catalog );       // Product category thumbs    /* update_option( 'shop_single_image_size', $single );      // Single product image    update_option( 'shop_thumbnail_image_size', $thumbnail );   // Image gallery thumbs */}add_action( 'after_switch_theme', 'yourtheme_woocommerce_image_dimensions', 1 );

You will need to paste this code snippet in function.php file of your active child theme or theme…

You can comment/uncomment the code (or remove some portions) to feet your needs. This code will overwrite define options in WooCommerce settings > Products > Display (Product Images).

ACTIVATION:
You will need to switch your active theme to another and then switch back to activate it.

References:

• Set WooCommerce image dimensions upon theme activation