A pattern will work. You just have to give the <image>
a size. Unlike HTML, SVG images default to width and height of zero.
Also, if you want the image to scale with the circle, then you should specify a viewBox
for the pattern.
<svg id="graph" width="100%" height="400px">
<!-- pattern -->
<defs>
<pattern id="image" x="0%" y="0%" height="100%" width="100%"
viewBox="0 0 512 512">
<image x="0%" y="0%" width="512" height="512" xlink:href="https://cdn3.iconfinder.com/data/icons/people-professions/512/Baby-512.png"></image>
</pattern>
</defs>
<circle id="sd" class="medium" cx="5%" cy="40%" r="5%" fill="url(#image)" stroke="lightblue" stroke-width="0.5%" />
</svg>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…