Image a la place du bouton classique

 <form action="index.php" method="post">
    Login : <input type="text" name="login" value="<?php if (isset($_POST['login'])) echo htmlentities(trim($_POST['login'])); ?>"><br />
    Mot de passe : <input type="password" name="pass" value="<?php if (isset($_POST['pass'])) echo htmlentities(trim($_POST['pass'])); ?>"><br />
    <input type="submit" name="connexion" value="Connexion">
    </form>

<input type="image" id="ok" src="images/ok.png" title="Connexion" style="float:right;padding:15px 8px 0 8px;height:23px"/></input> 
<form action="index.php" method="post" style="float:right;height:25px;padding:18px 0 0 0">
<div>
Login <input type="text" value="<?php if (isset($_POST['login'])) echo htmlentities(trim($_POST['login'])); ?>" style="width:100px" name="login" onclick="effacer(this, 'Pseudo');" onblur="restaurer(this, 'Pseudo');" />	
Mot de passe <input type="password" name="pass" style="width:100px" value="<?php if (isset($_POST['pass'])) echo htmlentities(trim($_POST['pass'])); ?>" onclick="effacer(this, 'motdepasse');" onblur="restaurer(this, 'motdepasse');" />
</div>
</form>