You cannot change the img src using css. You can use the following pure css solution though. 
HTML:
<div id="asks"></div>
CSS:
#asks {
  width: 100px;
  height: 100px;
  background-image: url('http://dummyimage.com/100x100/0000/fff');
}
#asks:hover {
  background-image: url('http://dummyimage.com/100x100/eb00eb/fff');
}
Or, if you don't want to use a div with a background image, you can use a javascript/jQuery solution. 
Html:
<img id="asks" src="http://dummyimage.com/100x100/000/fff" />
jQuery:
$('#asks')
  .mouseenter(function(){$('#asks').attr('src', 'http://dummyimage.com/100x100/eb00eb/fff');})
  .mouseleave(function(){$('#asks').attr('src', 'http://dummyimage.com/100x100/000/fff');});