Facing problem when adding Datamember (httppostedfile)

Question

Hi I am new to Wcf, I want to implement my frist web service, By this I want to upload images.
But here I am facing problem
when I adding 
[datamember] 
HttpPostedFile then its show this message. 
before this its working fine.



Now when i run this its show


So what is going wrong here................