I have PowerCharts v.3.2.1. The documentation says bgImage and related parameters were introduced in this version for the Radar Chart, but bgImage does not work. I even tried it using the sample Radar XML provided in the documentation, making only one change:
<chart caption='Radar Chart' anchorAlpha='0' bgAlpha='0,0' bgImage='pizza.jpg' >
Note that bgSWF does work. I've attached two screens, one successfully using bgSWF and one unsuccessfully using bgImage.