Replace WooCommerce default thumbnail placeholder

woocommerce.php

/*
* goes in theme functions.php or a custom plugin. Replace the image filename/path with your own :)
*/

add_action( 'init', 'custom_fix_thumbnail' );

function custom_fix_thumbnail() {
    add_filter('woocommerce_placeholder_img', 'custom_woocommerce_placeholder_img');

    function custom_woocommerce_placeholder_img( $src ) {
        $src = '<span class="thumb-placeholder"><i class="icon-camera"></i></span>';
        return $src;
    }

}