<use xlink:href="#imageElement" filter="url(#innerShadowElement)"/>
<def>
    <g id="imageElement">
         ...
    </g>
    <use id="shadowElement" xlink:href="#imageElement" transform="..."/>
    <filter id="innerShadowFilter" filterUnits="userSpaceOnUse">
        <feImage result="image" x="0" y="0" xlink:href="#shadowElement"></feImage>
        <feGaussianBlur in="image" stdDeviation="5" result="blur"></feGaussianBlur>
        <!-- Invert the alpha of the graphic and clip it to SourceAlpha -->
        <feComposite operator="out" in="SourceAlpha" in2="blur" result="inverse"></feComposite>
        <!-- The next two steps let us color the shadow -->
        <feFlood flood-color="black" flood-opacity=".5" result="color"></feFlood>
        <feComposite operator="in" in="color" in2="inverse" result="shadow"></feComposite>
        <!-- Place the shadow on top of the original image -->
        <feComposite operator="over" in="shadow" in2="SourceGraphic"></feComposite>
    </filter>
</def>