linked images also work with the xlink namespace:

<svg xmlns="http://www.w3.org/2000/svg xmlns:xlink="http://www.w3.org/1999/xlink class="tributary_svg"><rect width="200" height="200" x="100" y="100" fill="#1A9224"></rect><image width="200" height="200" xlink:href="https://twimg0-a.akamaihd.net/profile_images/2560186719/7f57... x="325" y="100"></image></svg>

d3 code to generate this http://enjalot.com/tributary/3747523/