add_action( 'sp_wpspro_before_product_thumbnail', 'wdr_woocommerce_sale_flash_custom');

function wdr_woocommerce_sale_flash_custom($posts){
	global $post; global $product;
	echo apply_filters( 'woocommerce_sale_flash', '<span class="onsale wdr-onsale">' . esc_html__( 'Sale!', 'woocommerce' ) . '</span>', $post, $product ); 
}