Add a background image (.png) to a SVG circle shape

Is this possible? The following is what I tried but it completely fills the circle with black.

<svg id='vizMenu' width="700" height="660">
    <defs>
        <filter id="dropshadow" height="130%">
            <feGaussianBlur in="SourceAlpha" stdDeviation="2"/> 
            <feOffset dx="0.5" dy="0.8" result="offsetblur"/> 
            <feMerge>
                <feMergeNode/>
                <feMergeNode in="SourceGraphic"/>
            </feMerge>
        </filter>
    </defs>
    <circle id='top' filter="url(#dropshadow)" cx="180" cy="120" r="80" stroke="#2E2E2E" stroke-width="2" fill="url('images/word-cloud.png')"/>
    <circle id='bottom' filter="url(#dropshadow)" cx="500" cy="300" r="80" stroke="#2E2E2E" stroke-width="2" fill="url('images/word-cloud.png')"/>
    <circle id='extra' filter="url(#dropshadow)" cx="180" cy="560" r="80" stroke="#2E2E2E" stroke-width="2" fill="#ffffff"/>
</svg>