You can pixelate images if you have the right "magic" displacementMap. Feel free to use the one referenced below (courtesy of Zoltan Fegyver).
Update: Changed the sample code to inline the displacementmap image as a data: URI (thanks for the code IllidanS4.)
The original answer had the displacementMap image hosted on a different domain. This used to work - but browsers implemented the new Filters security measures that disallow this. For production code today, you need the displacement map image served from the same domain as the source graphic's file or you need to inline the displacementMap.
<svg x="0px" y="0px" width="810px" height="600px" viewBox="0 0 810 600" color-interpolation-filters="sRGB">
<defs>
<filter id="pixelate" x="0%" y="0%" width="100%" height="100%">
<!--Thanks to Zoltan Fegyver for figuring out pixelation and producing the awesome pixelation map. -->
<feGaussianBlur stdDeviation="2" in="SourceGraphic" result="smoothed" />
<feImage width="15" height="15" xlink:href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAIAAAACDbGyAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAAWSURBVAgdY1ywgOEDAwKxgJhIgFQ+AP/vCNK2s+8LAAAAAElFTkSuQmCC" result="displacement-map" />
<feTile in="displacement-map" result="pixelate-map" />
<feDisplacementMap in="smoothed" in2="pixelate-map" xChannelSelector="R" yChannelSelector="G" scale="50" result="pre-final"/>
<feComposite operator="in" in2="SourceGraphic"/>
</filter>
</defs>
<image filter="url(#pixelate)" width="810" height="600" preserveAspectRatio="xMidYMid meet" xlink:href="http://uploads2.wikiart.org/images/vincent-van-gogh/the-starry-night-1889(1).jpg"/>
</svg>