How to locate this kind of html node,i am new to parse html.

Hi ,I want to parse a page like this ,

<div class = "parent">

        <div class = "child">  

                 <div class = "UniqueName1">

                   ..........

                  </div>

                 <img class = "IWantThis">

                   ..........

                  </div>

         </div>

        <div class = "child">

                 <div class = "UniqueName2">

                   ..........

                  </div>

                 <img class = "IWantThis">

                   ..........

                  </div>

        </div>

        <div class = "child">

                 <div class = "UniqueName3">

                   ..........

                  </div>

                 <img class = "IWantThis">

                   ..........

                  </div>

        </div>

</div>

  I want to get the img in IwantThis class  after class UniqueName3,how can i locate to that class,because the ID is unique,i have no idea,please help,Thanks, and sorry for my english