How to make user add images to drawable from system?

I'm facing a problem. I'm using an Image GridView of stuff, and I created an object and all. I want to make the user, by tapping a button, to add an image to resources, and then I want to even be able to use that image ID so i can add it to my objects and gridview.

This is my activity: (the part that is needed)

    public class ImageAdapter extends BaseAdapter {
    private Context mContext;

    public ImageAdapter(Context c) {
        mContext = c;
    }

    public int getCount() {
        return mThumbIds.size();
    }

    public Object getItem(int position) {
        return null;
    }

    public long getItemId(int position) {
        return 0;
    }

    // create a new ImageView for each item referenced by the Adapter
    public View getView(int position, View convertView, ViewGroup parent) {
        View grid;
        LayoutInflater inflater = (LayoutInflater) mContext
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (convertView == null) {
            grid = new View(mContext);
            grid = inflater.inflate(R.layout.grid_single, null);
            ImageView imageView = (ImageView)grid.findViewById(R.id.grid_image);
            TextView textView = (TextView) grid.findViewById(R.id.grid_text);
            imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
            textView.setText(mThumbIds.get(position).getDesc());
            imageView.setImageResource(mThumbIds.get(position).getImageID());
        } else {
            grid = (View) convertView;
        }
        return grid;
    }
}

public void createAll(){
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
    parentGender = prefs.getString("parentGender", "1");
    parentNum = prefs.getString("parentNum", "");
    childName = prefs.getString("childName", "Kiddo");
    final String opening=parentGender+", "+childName;
    if(mThumbIds.size()==0){
        mThumbIds.add(tv);
        mThumbIds.add(cookie);
        mThumbIds.add(toilet);
        mThumbIds.add(toys);
        mThumbIds.add(food);
        mThumbIds.add(water);
    }
    final GridView gridview = (GridView) findViewById(R.id.grid);
    gridview.setAdapter(new ImageAdapter(this));


    gridview.setOnItemClickListener(new AdapterView.OnItemClickListener() {


        public void onItemClick(AdapterView<?> parent, View v, final int position, long id) {

blah blah blah.....(from now on its the handling of a click on item)

NOTE::: mThumbIds is an ArrayList<E> E> of my object which contains a title and an image ID!

How do i make a user add an image and use its id??