This sample is to display the background image.

Image
source: the url of the image file
maintainAspectRation: Whether or not the aspect ratio is the same to the original image. 
If it is true, the chart will try to keep the resolution ratio of the original image otherwise, 
the chart will adjust the image to fit the size of the chart.
alpha: the transparency of the image.

<backgroundElements>
	<Image source="http://www.riamore.net/images/koolbg.jpg" 
	maintainAspectRatio="false" alpha="1"/>
</backgroundElements>