If you don’t have an image for every product in your shop. And you want to show for products without image some default image. Please follow these instructions:

Open files /goodstore/templates/woocommerce/content-product-X.php

And change on line 45 this:

 jwUtils::the_post_thumbnail('woo-size');

to this:

if (has_post_thumbnail()) {
   //get featured image
   jwUtils::the_post_thumbnail('woo-size');
} else {
   //default
   echo '<img src="YOUR_IMAGE_URL" alt="'.get_the_title().'" />';
}

For others (e.g. in widgets etc) please follow this link