The placeholder image can now be set at: WooCommerce > Settings > Products as of version 3.5x.
More info at: Configuring WooCommerce Settings.
WooCommerce 3.4x or earlier
Add code to your child theme’s
functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s
functions.php file as this will be wiped entirely when you update the theme.