How to set default product thumbnail image on WooCommerce even if there is an existing product image?

All right so let me explain the solution for you. You have to remove product loop original image thumbnail with the remove action. Then you have to insert new action which is calling the custom thumbnail. Don’t forget to replace URL OF YOUR IMAGE HERE with your image URL. For a single product you have to do the same. This code goes into functions.php file of your theme. Tested and works.

//Product loop

function custom_image_woo() {
    // Remove original product image from product loop
  remove_action( 'woocommerce_before_shop_loop_item_title', 'custom_image_woo', 10 );
    // Your custom image thumbnail function
    function here_is_the_magic() {
        echo '<img src="URL OF YOUR IMAGE HERE">';
    }
    add_action( 'woocommerce_before_shop_loop_item_title', 'here_is_the_magic', 10 );
}
add_action( 'woocommerce_init', 'custom_image_woo');

// Single product page

//this removes product featured image

add_filter('woocommerce_single_product_image_thumbnail_html', 'main_image_away', 10, 2);
function main_image_away($html, $attachment_id ) {
    global $post, $product;
    $featured_image = get_post_thumbnail_id( $post->ID );
    if ( $attachment_id == $featured_image )
        $html = '';
    return $html;
}

//this will add your image before gallery

function gallery(){
    echo '<img src="URL OF YOUR IMAGE HERE">';
}
add_action( 'woocommerce_product_thumbnails', 'gallery', 10 );