input type image

[merha]

<? if (isset($_REQUEST['sub'])) {?> {
bla bla blabla bla bla bla bla bla }

<form action="registra.php" method="post">
<input type="submit" value="registrati" name="sub">

se metto al posto di submit

<input type="image" src="foto.gif" name="sub" >

nn mi va lo script

why???