Espero te pueda servir a mi me funciono este codigo:
Código PHP:
$conn= db_connect();
$username= $_SESSION['valid_user'];
$nomimagen= $_FILES['sub']['name'];
if(move_uploaded_file($_FILES['sub']['tmp_name'], "/imagenes")) {
echo "The file ". basename( $_FILES['sub']['name']).
" has been uploaded";
$result=$conn->query("insert into imagenes value
('$username','$nomimagen')");
} else{
echo "There was an error uploading the file, please try again!";
}