¿Cómo puedo subir una imagen de Android a un servicio ASMX?

Tengo un problema sobre subir una imagen de mi aplicación Android a un servicio ASMX, mi versión de Android es 7.1.1 y mi servicio fue creado con Framework 4.5.

[WebMethod]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
    public void SaveImage()
    {
        if (!System.IO.Directory.Exists(HttpContext.Current.Server.MapPath("~/Upload")))
        {
            System.IO.Directory.CreateDirectory(HttpContext.Current.Server.MapPath("~/Upload/"));
        }

        string path = HttpContext.Current.Server.MapPath("~/Upload/").ToString();
        var Request = HttpContext.Current.Request;
        if (Request.Files.Count > 0)
        {
            var file = Request.Files[0];              
            file.SaveAs(path + file.FileName);               
        }           
    }

Aplicación Android:

/**
 * Created by SOPORTE on 05/12/2016.
 */

import android.app.DownloadManager;
import android.app.ProgressDialog;
import android.content.Intent;
import android.graphics.Bitmap;
import android.net.Uri;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Base64;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;

import com.android.volley.AuthFailureError;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Hashtable;
import java.util.Map;


public class upload extends AppCompatActivity implements View.OnClickListener {




    private Button buttonChoose;
    private Button buttonUpload;

    private ImageView imageView;

    private EditText editTextName;

    private Bitmap bitmap;

    private int PICK_IMAGE_REQUEST = 1;

    private String UPLOAD_URL ="http://192.168.0.18:8086/ServicesLogistica.asmx/UploadFile";


    private String KEY_IMAGE = "image";
    private String KEY_NAME = "name";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_upload);

        buttonChoose = (Button) findViewById(R.id.buttonChoose);
        buttonUpload = (Button) findViewById(R.id.buttonUpload);

        editTextName = (EditText) findViewById(R.id.editText);

        imageView  = (ImageView) findViewById(R.id.imageView);

        buttonChoose.setOnClickListener(this);
        buttonUpload.setOnClickListener(this);
    }

    public String getStringImage(Bitmap bmp){
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        bmp.compress(Bitmap.CompressFormat.JPEG, 100, baos);
        byte[] imageBytes = baos.toByteArray();
        String encodedImage = Base64.encodeToString(imageBytes, Base64.DEFAULT);
        return encodedImage;
    }

    private void uploadImage(){
        //Showing the progress dialog
        final ProgressDialog loading = ProgressDialog.show(this,"Uploading...","Please wait...",false,false);
        StringRequest stringRequest = new StringRequest(Request.Method.POST, UPLOAD_URL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String s) {
                        //Disimissing the progress dialog
                        loading.dismiss();
                        //Showing toast message of the response
                        Toast.makeText(upload.this, s , Toast.LENGTH_LONG).show();
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError volleyError) {
                        //Dismissing the progress dialog
                        loading.dismiss();

                        //Showing toast
                        Toast.makeText(upload.this, volleyError.getMessage().toString(), Toast.LENGTH_LONG).show();
                    }
                }){
            @Override
            protected Map<String, String> getParams() throws AuthFailureError {
                //Converting Bitmap to String
                String image = getStringImage(bitmap);

                //Getting Image Name
                String name = editTextName.getText().toString().trim();

                //Creating parameters
                Map<String,String> params = new Hashtable<String, String>();

                //Adding parameters
                params.put(KEY_IMAGE, image);
                params.put(KEY_NAME, name);

                //returning parameters
                return params;
            }
        };

        //Creating a Request Queue
        RequestQueue requestQueue = Volley.newRequestQueue(this);

        //Adding request to the queue
        requestQueue.add(stringRequest);
    }

    private void showFileChooser() {
        Intent intent = new Intent();
        intent.setType("image/jpeg");
        intent.setAction(Intent.ACTION_GET_CONTENT);
        startActivityForResult(Intent.createChooser(intent, "Select Picture"), PICK_IMAGE_REQUEST);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == PICK_IMAGE_REQUEST && resultCode == RESULT_OK && data != null && data.getData() != null) {
            Uri filePath = data.getData();
            try {
                //Getting the Bitmap from Gallery
                bitmap = MediaStore.Images.Media.getBitmap(getContentResolver(), filePath);
                //Setting the Bitmap to ImageView
                imageView.setImageBitmap(bitmap);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void onClick(View v) {

        if(v == buttonChoose){
            showFileChooser();
        }

        if(v == buttonUpload){
            uploadImage();
        }
    }
}