I need to show/hide an image based on the result from 2 fields in ssrs

Question

I need to show or hide an image based on the result from multiplying 2 fields. Below is my code in SSRS but with no luck.

 

=iif(sum(Fields!SumInsured.Value * Fields!ExRate.Value <= 500000000), false, true)

I hope someone can help me with this. Thanks in advance.