By default the WooCommerce product slider will move up and down to match the height of the currently displayed picture. You can disable this by hooking into the "woocommerce_single_product_carousel_options" filter and setting the "smoothHeight" option to false.
add_filter( 'woocommerce_single_product_carousel_options', function( $options ) {
$options['smoothHeight'] = false;
return $options;
} );