Essaye ça :
<?php
//-----------------------------------------------
//DECLARE LES VARIABLES
//-----------------------------------------------
$email_expediteur='votre_mail@fai.fr';
$email_reply='email_de_reponse@fai.fr';
$message_texte='Bonjour,'."n\n".'Voici un message au format texte';
$message_html='<html>
<head>
<title>Titre</title>
</head>
<body>
<p>Test de message</p>
</body>
</html>';
//-----------------------------------------------
//GENERE LA FRONTIERE DU MAIL ENTRE TEXTE ET HTML
//-----------------------------------------------
$frontiere
= '-----=' . md5(uniqid(mt_rand()));
//-----------------------------------------------
//HEADERS DU MAIL
//-----------------------------------------------
$headers = 'From: "Anthony Rossetto" <'.$email_expediteur.'>'."\n";
$headers .= 'Return-Path: <'.$email_reply.'>'."\n";
$headers .= 'MIME-Version: 1.0'."\n";
$headers .= 'Content-Type: multipart/mixed; boundary="'.$frontiere.'"';
//-----------------------------------------------
//MESSAGE TEXTE
//-----------------------------------------------
$message = 'This is a multi-part message in MIME format.'."\n\n";
$message .= '--'.$frontiere."\n";
$message .= 'Content-Type: text/plain; charset="iso-8859-1"'."\n";
$message .= 'Content-Transfer-Encoding: 8bit'."\n\n";
$message .= $message_texte."\n\n";
//-----------------------------------------------
//MESSAGE HTML
//-----------------------------------------------
$message .= '--'.$frontiere."\n";
$message .= 'Content-Type: text/html; charset="iso-8859-1"'."\n";
$message .= 'Content-Transfer-Encoding: 8bit'."\n\n";
$message .= $message_html."\n\n";
//-----------------------------------------------
//PIECE JOINTE
//-----------------------------------------------
$message .= '--'.$frontiere."\n";
$message .= 'Content-Type: image/jpeg; name="nom_du_fichier.jpg"'."\n";
$message .= 'Content-Transfer-Encoding: base64'."\n";
$message .= 'Content-Disposition:attachement; filename="nom_du_fichier.jpg"'."\n\n";
$message .= chunk_split(base64_encode(file_get_contents('nom_du_fichier.jpg')))."\n";
mail($destinataire,$sujet,$message,$headers);
?>