So you can enable Custom Fields by clicking “Screen Options” at the top right of the edit area, and checking the “Custom Fields” checkbox.
Then add a new Custom Field, and give it a name like: sideheader
Then in the value, enter the ID of the image you want to use: 123
Then your code would be:
<?php
$custom_image = get_post_meta( get_the_ID(), 'sideheader', true );
if ( function_exists( 'is_product' ) && is_product() && $custom_image ) {
echo wp_get_attachment_image( $custom_image, 'full', array( 'class' => 'aligncenter' ) );
}
?>