The right-click method (Save as image...) will only be available if your dashboard is using the FlashChart module. If you are using simple XML and no special chart properties, then you are using JSchart and you will not be able to save the chart as an image. For more information about the differences, see Advanced charting options in the documentation.
You can convert your chart to advanced XML, or if you're already using advanced XML you could change the charting module. See this related Answers posting for confirmation.