Load images from sdcard in grid with Picasso

I have a grid that has string arraylist of items:

    final GridView grid = (GridView) view.findViewById(R.id.gridview);
    final ArrayList<String> items = new ArrayList<String>();

    items.add("1a");
    items.add("1b");
    items.add("1c");
    items.add("1d");
    items.add("1e");
    items.add("2a");
    items.add("2b");
    items.add("2c");
    items.add("2d");
    items.add("2e");
    items.add("3a");
    items.add("3b");
    items.add("3c");
    items.add("3d");
    items.add("3e");
    items.add("3f");
    items.add("4a");
    items.add("4b");
    items.add("4c");
    items.add("4d");
    items.add("4e");
    items.add("4f");
    items.add("4g");

Later, I tried to do something with onClick, selecting a certain position:

 grid.setAdapter(new GridAdapter(items));

        grid.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
                  if(position == 3)
                      ...
            }
        });

What I would like is:

I have like 20 pictures in my res/drawable folder. I'd like to have a fragment that contains an imageView, that runs on onClick, and loads picture named 1a.png into item 1a, 1b.png into item 1b and so forth, without having selecting certain positions.

So when I click on 1a in gridview, it displays 1a.png, 1b displays 1b.png and so forth...

Thanks.