Probá así:
Código PHP:
<?php
if(isset($_POST['Submit'])){
echo '<pre>';
print_r($_POST);
echo '</pre>';
exit;
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body>
<form id="form1" name="form1" method="post" action="<?php echo $_SERVER['PHP_SELF'] ?>" onsubmit="s=document.getElementById('imagenes');s.setAttribute('multiple','multiple');s.setAttribute('size',1);for(i=0;i<s.options.length;i++)s.options[i].selected=1">
<select name="imagenes[]" id="imagenes">
<option value="001.jpg">001.jpg</option>
<option value="002.jpg">002.jpg</option>
<option value="003.jpg">003.jpg</option>
<option value="004.jpg">004.jpg</option>
</select>
<input type="submit" name="Submit" value="Enviar" />
</form>
</body>
</html>