step 1 :  add a class in your web service project with following details

[DataContract]
    public class FilesUploader
    {

        [DataMember]
        public string FileName { get; set; }

        [DataMember]
        public string FoldersName { get; set; }

        [DataMember]
        public byte[] Imagestream { get; set; }


    public string Upload(FilesUploader image)
        {
            FileStream fileStream = null;
            BinaryWriter writer = null;
            try
            {
                //filePath = HttpContext.Current.Server.MapPath(".") + ConfigurationManager.AppSettings["PictureUploadDirectory"] + image.ImageName;
                string location = ConfigurationManager.AppSettings["imageFileLocation"].ToString();
                string strFilePath = location + image.FoldersName + "\\" + image.FileName;
            
                if (image.FileName != string.Empty)
                {
                    fileStream = File.Open(strFilePath, FileMode.Create);
                    writer = new BinaryWriter(fileStream);
                    writer.Write(image.Imagestream);
                }
                return OperatioinResult.Success;
            }
            catch (Exception ex)
            {
                return OperatioinResult.ERROR + " ~ " + ex.Message;
            }
            finally
            {
                if (fileStream != null)
                    fileStream.Close();
                if (writer != null)
                    writer.Close();
            }
        }


}


step 2 : add this method in your service operation contracnt 

 [OperationContract]
        public string FilesUploader_UploadFiles(FilesUploader fileByte)
        {
            //MemoryStream byFile = new MemoryStream(System.Text.Encoding.ASCII.GetBytes(fileByte));
            return new FilesUploader().Upload(fileByte);
        }


Step 3 : use this service reference in your silverlight clinet project.

           OpenFileDialog op = new OpenFileDialog();
           op.Filter = "Images (*.jpg, *.png, *.bmp)|*.jpg;*.png;*.bmp";
            op.ShowDialog();
            if (op.File != null && op.File.Name != "")
            {
                    fileStram = (FileStream)op.File.OpenRead();
                    byte[] bytes = new byte[fileStram.Length];
                    fileStram.Read(bytes, 0, (int)fileStram.Length);

                    FilesUploader fUploader = new FilesUploader();
                    tempImage = "tempPatientImages" + op.File.Extension;
                    fUploader.FileName = tempImage;
                    fUploader.Imagestream = bytes;
                    fUploader.FoldersName = "PatientImges";

                    service.FilesUploader_UploadFilesAsync(fUploader);
                    // service is the object of service provider you need it chage it to your object name.
}