Well i had look at it and found that GetROProperty function works only on the properties that are defined in the help for that object. QTP has a mapping for some of the actual dom properties to the RO properties. And object spy shows all the DOM properties related to that object.
idx=Browser("micClass:=Browser").Page("micClass:=P age").Image("src:=.*search_y.gif").GetROProperty(" sourceindex")
will never work becuase QTP does not have any mapping to the actual DOM.
but this will surely do