$('.gallery img')[i].attr('id', 'img' + i);

$('.gallery img').each(function(i) {
    $(this).attr('id', 'img' + i);
    // You can also add more code here if you wish to manipulate each IMG element further
});

$('.gallery img')[i]

$('.gallery img').eq(i).attr('id', 'img' + '"' + i + '"');

$('.gallery img').each(function(index, elem) {
  $(this).attr('id', 'img' + index);
});