Bootstrap Vertical Alignment of an image

Question

I'm facing a difficulty in alignmnet of the images. I searched a lot and saw similar threads but none solved my problem.

I want to align the images in the center of the screen but I cannot.

<div class="container-fluid ">


   <!--olokliro to div clickable, me blur pantou-->
    <a href="http://2013.tedxuniversityofmacedonia.com">

     <div id="dare" class="col-xs-12 col-sm-6 col-md-3  ">

        <img src="images/dare new rotated.png" class="img-responsive center-block" alt="Dare" >

     </div>
    </a>

    <a href="http://2014.tedxuniversityofmacedonia.com"> 
     <div id="ithacas" class="col-xs-12 col-sm-6 col-md-3  ">

       <figure><img src="images/ithacas rotate.png" class="img-responsive " alt="Ithacas"><figure>

     </div>
    </a>

    <a href="http://2015.tedxuniversityofmacedonia.com">
     <div id="blindspot" class="col-xs-12 col-sm-6 col-md-3  ">
       <figure><img src="images/blind-spot rotate.jpg" class="img-responsive" alt="Blind spot"></figure>
     </div>
    </a>

    <a href="http://blog.tedxuniversityofmacedonia.com">
     <div id="comingsoon" class="col-xs-12 col-sm-6 col-md-3  ">
       <figure><img src="images/blog.jpg" class="img-responsive" alt="Coming Soon"><figure>
     </div>
    </a>


  </div>

And the output is this