You can Use a "Mask" Image.. this image should be a triangle in your case (or any other shape you need) like this :
ImageView mImageView= (ImageView)findViewById(R.id.imageview_id);
Bitmap original = BitmapFactory.decodeResource(getResources(),R.drawable.contentimage);
Bitmap mask = BitmapFactory.decodeResource(getResources(),R.drawable.mask);
Bitmap result = Bitmap.createBitmap(mask.getWidth(), mask.getHeight(), Config.ARGB_8888);
Canvas mCanvas = new Canvas(result);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
mCanvas.drawBitmap(original, 0, 0, null);
mCanvas.drawBitmap(mask, 0, 0, paint);
paint.setXfermode(null);
mImageView.setImageBitmap(result);
mImageView.setScaleType(ScaleType.CENTER);
mImageView.setBackgroundResource(R.drawable.background_frame);