hoge

gistfile1.txt

before:
list.append('<div class="one-fourth" style="margin-top:15px;"><a href="'+h+'" '+s.attr+'  title="'+photo['title']+'" target="_blank"><img src="'+t+'" alt="'+photo['title']+'" /></a></div>');

after:
var element = $('<div class="one-fourth" style="margin-top:15px;"><a href="'+h+'" '+s.attr+'  title="'+photo['title']+'" target="_blank"><img src="'+t+'" alt="'+photo['title']+'" /></a></div>');
if(i % 4 === 0){
  element.addClass('column-first');
}else if(i % 4 === 3){
  element.addClass('column-last');
}
list.append(element);