how to setup backgroud imagge based on the expression

Hi

 I develop an ssrs report, the requirement is if the copy is orginal i need to show the back ground image on that report or i should not show that image,

 i create tow image ,one is log and another one is empty image.

i setup expression but i could not save.. please help me

The value of the BackgroundImageValue property for the report section ‘ReportSection0’ is “Iif(Parameters!P_IsDuplicate.Value="true","Original_mo","Emptyimage")”, which is not a valid BackgroundImageValue.

this is the error i am getting , both are embedded image

Thanks & Regards Manoj