I saw this Viz online:
-they are using the funnel image (bottom left) as a filter
Applying the Power of Tableau to Salesforce.com - YouTube
I too have different stages of the sale and would like to do something similar (currently, we just pull down names from filter dropdown)
They don't actually show it being used as a filter, but you could possibly dummy up some data and set it up to do that with dashboard actions. Remember that it's not an image, it's another worksheet that is placed & coloured correctly.
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