<div class="row org-logo m-b-lg">
<div class="col-sm-3 col-xs-6 text-center">
<a href="http://www.psychologicalscience.org/" target="_blank"><img src="/static/img/meeting-page/APS.jpg" alt="Logo for American Physiological Society (APS)" class="img-responsive"></a>
</div>
<div class="col-sm-3 col-xs-6 text-center">
<a href="http://www.bitss.org/" target="_blank"><img src="/static/img/meeting-page/BITSS.png" alt="Logo for Berkeley Initiative for Transparency in the Social Sciences (BITSS)" class="img-responsive"></a>
</div>
<div class="col-sm-3 col-xs-6 text-center">
<a href="http://www.nrao.edu/" target="_blank"><img src="/static/img/meeting-page/NRAO.jpg" alt="Logo for National Radio Astronomy Observatory (NRAO)" class="img-responsive"></a>
</div>
<div class="col-sm-3 col-xs-6 text-center">
<a href="http://www.spsp.org/" target="_blank"><img src="/static/img/meeting-page/SPSP.jpg" alt="Logo for Society for Personality and Social Psychology (SPSP)" class="img-responsive"></a>
</div>
</div>
https://i.sstatic.net/Gzy7U.png
In the displayed row of organizational logos, there is an alignment issue with the APS logo. To center all the images despite their varying sizes, what steps should be taken?