Replace default placeholder - smartpost

add_filter( 'pcp_no_thumb_placeholder', 'custom_pcp_placeholder_image' );

function custom_pcp_placeholder_image( $placeholder_img ) {
	// Set your custom placeholder image URL.
	$placeholder_img = 'https://yourwebsite.com/wp-content/uploads/custom-placeholder.png';
	return $placeholder_img;
}