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

Question

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>

Answer 1

An image fill for an svg element is achieved through SVG Patterns...
<svg width="700" height="660">
<defs>
<pattern id="image" x="0" y="0" patternUnits="userSpaceOnUse" height="1" width="1">
<image x="0" y="0" xlink:href="url.png"></image>
</pattern>
</defs>
<circle id='top' cx="180" cy="120" r="80" fill="url(#image)"/>
</svg>

Answer 2

Well, I couldn't make it work with the accepted answer. This is how I ended up doing it:
<svg width="100" height="100">
<defs>
<pattern id="image" patternUnits="userSpaceOnUse" height="100" width="100">
<image x="0" y="0" height="100" width="100" xlink:href="http://i.imgur.com/7Nlcay7.jpg"></image>
</pattern>
</defs>
<circle id='top' cx="50" cy="50" r="50" fill="url(#image)"/>
</svg>
If you want to customize the size, use this as a scale reference:
x = yourPreferredSize
<svg width=">2x" height=">2x">
<defs>
<pattern id="image" patternUnits="userSpaceOnUse" height=">2x" width=">2x">
<image x="0" y="0" height="2x" width="2x" xlink:href="http://i.imgur.com/7Nlcay7.jpg"></image>
</pattern>
</defs>
<circle id='top' cx="x" cy="x" r="x" fill="url(#image)"/>
</svg>
(This scale works for squared images)

Answer 3

Image repetition problem solved with proper explanation (Thanks to AmeliaBR)
TL;DR: The concept of objectBoundingBox and preserveAspectRatio are used!
<svg height = "10%" width = "10%">
<defs>
<pattern id = "attachedImage" height = "100%" width = "100%"
patternContentUnits = "objectBoundingBox">
<image xlink:href = "url.png" preserveAspectRatio = "none"
width = "1" height = "1"/>
</pattern>
</defs>
<circle cx = "50%" cy = "50%" r = "35%" fill = "url(#attachedImage)"/>
</svg>

Answer 4

I know this is an old question, but I used a filter to overlay the image. The above solution didn't work for me because of scaling and it seemed like the images was tiled. I used this instead, I hope it will help others as well.
<svg width="700" height="660">
<filter id="this_image" x="0%" y="0%" width="100%" height="100%">
<feImage xlink:href="test_image.png"/>
</filter>
<circle filter="url(#this_image)" cx="180" cy="120" r="80" />
</svg>

Answer 5

This is my solution, the differences are that this doesn't use the patternUnits="userSpaceOnUse" and that you specify the desired width and height of the image element.
<defs>
<pattern id="{some name}" x="0" y="0" width="1" height="1">
<image href="{image url}" x="0" y="0" width="{desired width}" height="{desired height}"></image>
</pattern>
</defs>