//proceso de transformar la imagen BitMap en un String:
//android:src="c:logo.png"
Resources r = this.getResources();
Bitmap bm = BitmapFactory.decodeResource(r, R.drawable.logo);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
bm.compress(Bitmap.CompressFormat.PNG, 100, baos); //bm is the bitmap object
byte[] b = baos.toByteArray();
//String encodedImage = Base64.encode(b, Base64.DEFAULT);
encodedImage = Base64.encodeBytes(b);

byte[] decodedString = Base64.decode(encodedImage, Base64.DEFAULT);
Bitmap decodedByte = BitmapFactory.decodeByteArray(decodedString, 0, decodedString.length);

byte[] decodedString = Base64.decode(encodedImage, Base64.URL_SAFE);
Bitmap decodedByte = BitmapFactory.decodeByteArray(decodedString, 0, decodedString.length);