You can use net.rim.device.api.system.EncodedImage class to scale down an image
Code:
EncodedImage ei = EncodedImage.getEncodedImageResource("image.png");
ei.setScale(2); //half the size
BitmapField bmp = new BitmapField(ei.getBitmap());
add(bmp);
But I don't think this class supports resizing to any arbitrarly size. The only other way I can think of is overriding the paint() method of the BitmapField class and then using Graphics.drawBitmap... Well, other than using photoshop