Вот рабочий код index.php:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">
<html><head><title>New</title>
<style type="text/css">
#img {
position: absolute;
z-index: 1;
top: 50px;
left: 50px;
cursor: pointer;
width: 50px;
}
</style>
</head><body>
<form action="" method="post">
<input type="hidden" name="inbut" value="120"/>
<input id="img" type="image" src="http://i86.carguru.ru/6/79/17906/59/224459/brp28.gif" alt="картинка" title="я отправляю методом POST"/>
</form>
<?php echo $_POST['inbut'];?>
</body></html>