how to get dynamic generated src attribute of web element?

I want to check if picture or flash has successfully displayed on web page.But the element can not be found in page source.

I get the element in chrome Developer tools.

<div id="QQcom_all_Rectangle:1" data-loc="QQcom_all_Rectangle" data-index="1" style="height: 250px; display: block; width: 300px; position: relative;" class="l_qq_com" adconfig_lview="l.qq.com" adconfig_charset="gbk" adconfig_lview_template="http://ift.tt/1legTWP}" oid="1800716433" display="banner">
   <a href="http://ift.tt/1M12ZzM" target="_blank" style="display:block;cursor:pointer;width:300px;height:250px;background-image:url(http://ift.tt/1legVha);background-size:300px 250px;filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='http://ift.tt/1legVha',sizingMethod='scale');"></a>
   <a class="absolute a_cover" href="http://ift.tt/1M12XYO" target="_blank" rel="nofollow" style="position:absolute;width:300px;height:250px;left:0px;top:0px;cursor:pointer;z-index:10;background-color:#fff;filter:alpha(opacity=0);opacity:0;"></a>
   <div class="absolute" style="position: absolute; width: 24px; height: 16px; left: 26px; bottom: 0px; cursor: pointer; display: none; z-index: 20; background: url(http://ift.tt/1legUd5) 50% 0% no-repeat;"></div>
   <div style="position: absolute; left: 0px; bottom: 0px; width: 26px; height: 16px; z-index: 12; background: url(http://ift.tt/1M12XYQ) 100% 0% no-repeat;"></div>
   <div class="absolute" style="position:absolute;width:18px;height:18px;right:0px;bottom:0px;cursor:pointer;z-index:20;background:url(http://ift.tt/1legVhe) no-repeat;"></div>
</div>

but in page source,it only shows

<!--$loc$_div AD begin...."l=$loc$&log=off"--><div id="QQcom_all_Rectangle:1"  data-loc="QQcom_all_Rectangle" data-index="1" style="height:0;" class="l_qq_com"></div><!--$loc$ AD end --><!--[if !IE]>|xGv00|c5668531d36ed7899852180841ca2aa2<![endif]-->  

how can I get the 300px_250px image url?
anyone knows?