the type you are using is wrong, you need to convert your data to byte[], and use SqlDbType.Image, see
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=uyWJkqueCHA.508%40tkmsftngp12also see
HOW TO: Read and Write BLOB Data by Using ADO.NET with Visual C# .NET
http://support.microsoft.com/default.aspx?scid=kb;EN-US;309158