Set wallpaper from gallery

 final int RESULT_LOAD_IMAGE=100;

On button Click 

Intent i = new Intent(Intent.ACTION_PICK,
                        android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
        startActivityForResult(i, 100);

@Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        // TODO Auto-generated method stub
         if (requestCode == RESULT_LOAD_IMAGE && resultCode == RESULT_OK && null != data) {
               String wallpaper = selectedWallpaperImage(getApplicationContext(),data);
               WallpaperManager wallpaperManager = WallpaperManager.getInstance(this);
             
               try {
    wallpaperManager.setBitmap(BitmapFactory.decodeFile(wallpaper));
                             //this line of code will set the selected image as your wallpaper...........
    } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
    }}
    }

public static String selectedWallpaperImage(Context context,Intent data){
             Uri selectedImage = data.getData();
     String[] filePathColumn = { MediaStore.Images.Media.DATA };

     Cursor cursor = context.getContentResolver().query(selectedImage,
             filePathColumn, null, null, null);
     cursor.moveToFirst();

     int columnIndex = cursor.getColumnIndex(filePathColumn[0]);
     String picturePath = cursor.getString(columnIndex);
     cursor.close();
return picturePath;
}

and add permission  <uses-permission android:name="android.permission.SET_WALLPAPER" />

And if you want direct set image from application resource 

WallpaperManager myWallpaperManager = WallpaperManager
            .getInstance(getApplicationContext());
          try {
           //set wallpaper picture from resource here
           myWallpaperManager.setResource(R.drawable.andr_gp);
           Toast.makeText(getApplicationContext(),"Success set as wallpaper",22).show();
          } catch (IOException e) {
              Toast.makeText(getApplicationContext(),"Error set as wallpaper",22).show();
          }