to answer your question:
// you can call this from inside the ListActivity Drawable dr = new BitmapDrawable(bitmap); // that's the bitmap you downloaded from your server getListView().setBackground(dr);
to give extra advice for a noob:
DO NOT use
StrictMode.enableDefaults(); with that you're masking out a very good protection against bad code. Learn how to do it properly. AsyncTask is a very popular and very easy to use threading platform and you should use it. And if you feel confident and adventurous use Loaders which are a bit more complex but far more powerful!