I pasted your expression into a sample report set up with a parameter Date, data type Date/Time, and got the same error when using the expression.
I resolved this by changing the data being compared to the parameter in the expression into a string that can readily be converted to a date, so from 11.04.2013
to "11-Apr-2013"
:
=iif(Parameters!Date.Value <> "11-Apr-2013", "http://rack.0.mshcdn.com/media/ZgkyMDEyLzEyLzA0L2I1L3doZXJlZG9nb29nLmJoTi5qcGcKcAl0aHVtYgk5NTB4NTM0IwplCWpwZw/4931e287/304/where-do-google-doodles-come-from--ff2932470c.jpg", " ")
This stopped the error occurring for me and worked as expected when I selected different dates; i.e. the URL on all dates except 11-Apr-2013.