Well I don't know how hard you tried on the feImage,. but this code works perfectly. You want to pull in the feImage, then clip it to the source with a feComposite "in". Then you can composite the dropshadow under that result.
<svg>
<defs>
<filter id="dropshadow" width="130%" height="130%">
<feGaussianBlur in="SourceAlpha" stdDeviation="3"/>
<feOffset dx="0" dy="4" result="offsetblur"/>
<feComponentTransfer in="offsetblur" result="dropshadow">
<feFuncA type="linear" slope="0.1"/>
</feComponentTransfer>
<feImage result="bgpic" xlink:href="http://www.sencha.com/img/sencha-large.png" />
<feComposite operator="atop" in="bgpic" in2="SourceGraphic" result="coikelwimg"/>
<feMerge>
<feMergeNode in="coikelwimg"/>
<feMergeNode in="dropshadow"/>
</feMerge>
</filter>
</defs>
<circle cx="50%" cy="50%" r="49%" filter="url(#dropshadow)" fill="#f8f8f8" stroke="#e7e7e7" stroke-width="1px"/>
</svg>