The charts do not support this. If you are a brave and enterprising soul, you can dive into the SVG and try to add them yourself. You'll need something like jQuery to get at the contents of the iframe the charts are built in (and I believe there is an SVG plugin for jQuery as well).
This is no easy task, and I wish you the best of luck.
Personally, I found it easier to layer images or divs on top of the chart and position them with CSS.