Try this:
// convert from bitmap to byte array
public byte[] getBytesFromBitmap(Bitmap bitmap) {
ByteArrayOutputStream stream = new ByteArrayOutputStream();
bitmap.compress(CompressFormat.JPEG, 70, stream);
return stream.toByteArray();
}
// get the base 64 string
String imgString = Base64.encodeToString(getBytesFromBitmap(someImg),
Base64.NO_WRAP);