Assuming that your image data is in a String called myImageData, the following should do the trick:
byte[] imageAsBytes = Base64.decode(myImageData.getBytes(), Base64.DEFAULT);
ImageView image = (ImageView)this.findViewById(R.id.ImageView);
image.setImageBitmap(
BitmapFactory.decodeByteArray(imageAsBytes, 0, imageAsBytes.length)
);
For Base64 decoding, you can use http://iharder.sourceforge.net/current/java/base64/ as Android doesn't contain Base64-support prior to 2.2.
Note, I didn't actually run this code, so you'll have to doublecheck for errors.
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