The key is setting useHTML to true: http://jsfiddle.net/highcharts/8FSp6/.
Here’s another approach, allowing true SVG images: http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/studies/axis-label-images/. The advantage of using SVG is that these images willl be visible in the exported chart as well.
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Using the SVG approach, is there a way to achieve the same with polar coordinates?