простое решение будет
private int PicPosition;
private Handler handler = new Handler();
...
...
...
private Runnable runnable = new Runnable() {
public void run() {
myslideshow();
handler.postDelayed(this, 1000);\\execute every second.
}
};
private void myslideshow()
{
PicPosition = gallery.getSelectedItemPosition() +1;
if (PicPosition >= Pictures.size())
PicPosition = gallery.getSelectedItemPosition(); //stop
else
gallery.setSelection(PicPosition);//move to the next gallery element.
}