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>