p.der {
    text-align: right;
	float:right;

}
p.izq {
    text-align: right;
	float:left;

}



<p class="der">
<a href="http://validator.w3.org/check?uri=referer">
<img src="http://www.w3.org/Icons/valid-xhtml10" alt="Valid XHTML 1.0 Strict" height="31" width="88" /></a>
</p>

<p class="izq">
<a href="http://jigsaw.w3.org/css-validator">
    <img style="border:0;width:88px;height:31px"
        src="http://jigsaw.w3.org/css-validator/images/vcss-blue"
        alt="¡CSS Válido!" />
    </a>
</p>