The class SubsamplingScaleImageView has also a method to load an image that isn't from Assets:
SubsamplingScaleImageView.setImageFile(String extFile)
So you can get the image from the URL and save it on internal storage. Then you can load the image using the path from internal storage. To get the image as a Bitmap from an URL:
URL myFileUrl = new URL (StringURL);
HttpURLConnection conn = (HttpURLConnection) myFileUrl.openConnection();
conn.setDoInput(true);
conn.connect();
InputStream is = conn.getInputStream();
Bitmap bm = BitmapFactory.decodeStream(is);
To save the Bitmap on internal storage:
FileOutputStream out = new FileOutputStream(context.getCacheDir() + filename);
bm.compress(Bitmap.CompressFormat.PNG, 90, out);
// Don't forget to add a finally clause to close the stream
Finally
SubsamplingScaleImageView imageView = (SubsamplingScaleImageView)findViewById(R.id.imageView);
imageView.setImageFile(context.getCacheDir() + filename);