Try this..
session_start();
// Define session fields into simple variables
$username = $_SESSION['username'];
$msg = "";
//db connection
$connection=mysql_connect ("connection","username","password") or die ("Cannot connect to the database at this time. Please go back and try again.");
mysql_select_db ("mydatabase", $connection) or die ("Couldn't select database. Please go back and try again.");
// check if vehicle image already exists
$sql_vimage_check = mysql_query("SELECT vimage FROM membersrigs WHERE Username='".$username."');
$vimage_check = mysql_num_rows($sql_vimage_check);
//perform sql database check
if($vimage_check > 0){
$msg = "You have already uploaded and image! Sorry!<br />";
$_SESSION = array();
session_destroy();
header("Location: members/add.php");
} else {
$msg = "Please upload your vehicle image";
include('upload.php'); //upload form then shows
exit(); // exit the script so that we do not create this account!
}
try making the Location: members/add.php absolute.