If you have your image resource in drawable folder you can refer them as follows:
If you bundle them in assets, then you will have to read these files as streams using AssetManager and then create a Bitmap from these stream using BitmapFactory. After this is done, you can set this Bitmap as background property of your View or ViewGroup in code.
Putting an image resource in drawable folder is NOT JUST copying an image file to a folder in the Project's structure.(Read official documentation on Application Resources). Android tools(aapt et al.) do a lot of stuff behind the scenes.