Try by changing your code as below.
int filelength = FileUpload1.PostedFile.ContentLength;
byte[] imagebytes = new byte[filelength];
FileUpload1.PostedFile.InputStream.Read(imagebytes,0,filelength);
SqlConnection cn = new SqlConnection(str);
SqlCommand cmdinsimg = new SqlCommand("dbo.upload_image", cn);
cmdinsimg.CommandType = CommandType.StoredProcedure;
SqlParameter[] prms = new SqlParameter[1];
prms[0] = new SqlParameter("@Property_Img",SqlDbType.Image);
prms[0].Value = imagebytes;
cmdinsimg.Parameters.Add(prms);
cn.Open();
cmdinsimg.ExecuteNonQuery();
cn.Close();