Ciao, per questo problema sarebbe piu' adatta la sezione htmnl/css

Ad ogni modo per spostarlo al centro potresti fare cosi':
codice:
<!DOCTYPE HTML>

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

<title>Documento senza titolo</title>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

<title>Caricamento</title>



<style>

body{text-align:center;}

.container {margin:auto; padding-top:50px;}

</style>



</head>



<body background="cuore2.jpg">

<div class="container">





<form action="upload.php" method="post" enctype="multipart/form-data">

<input type="file" name="upfile">

<input type="hidden" name="MAX_FILE_SIZE" value="1000000">

<input type="submit" value="Invia il file">

</form>

</div>

</body>

</html>