title: change class on different li's
body: <p>I have 18 images that gets displayed like 6 images in each row. I want to assign different class to every second last and last image in a row.
  eg :- </p>
  
  <pre><code>1    2    3    4    5    6
  7    8    9    10   11   12
  13   14   15   16   17   18
  </code></pre>
  
  <p>Here class should be different for the  <code>5   6    11    12    17    18</code></p>
  
  <p>I am able to make equation for this as leaving 5   6</p>
  
  <p>it could be <code>5n + (n-1)          6n</code>
  starting from n=2</p>
  
  <pre><code>   5(2) + (2-1)    6(2)  =    11    12
     5(3) + (3-1)    6(3)  =    17    18
  </code></pre>
  
  <p>I am not sure how to implement this using jquery.
  AS 5   6   should be as it is and then equation for 11,12,17,18 and starting n=2</p>
  
  <pre><code>&lt;li&gt;&lt;a href="#"&gt;&lt;span&gt;&lt;/span&gt;&lt;img src="images/img14.jpg" alt="" width="136" height="136" /&gt;&lt;em class="popup"&gt; &lt;strong class="arrow"&gt;&lt;/strong&gt;&lt;strong class="title"&gt;Sponsor Name Here&lt;/strong&gt;Lorem ipsum dolor sit amet, consectetur edt adipiscing elit. Nullam dignissim enim ut co. Lorem ipsum dolor sit amet, consectetur bel adipiscing elit nullam digniss&lt;/em&gt;&lt;/a&gt;&lt;/li&gt;
  </code></pre>
  
  <p>In em I want to add class popup-left </p>