‘border-radius’ doesn’t apply to svg:image elements (yet anyway). A workaround would be to create a rect with rounded corners, and use a clip-path.
The relevant part of the source:
<defs>
<rect id="rect" x="25%" y="25%" width="50%" height="50%" rx="15"/>
<clipPath id="clip">
<use xlink:href="#rect"/>
</clipPath>
</defs>
<use xlink:href="#rect" stroke-width="2" stroke="black"/>
<image xlink:href="boston.jpg" width="100%" height="100%" clip-path="url(#clip)"/>
It’s also possible to create several rect elements instead of using <use>
.