Upload your icon to the Atahualpa images folder and put the following in ATO>Add HTML/CSS Inserts>CSS Inserts. Of course changing "your_image.png" to the name of your png or jpg (I would suggest a png) and you may have to tweak the padding for your image.
HTML Code:
.post-headline {
background: url('<?php echo get_template_directory_uri(); ?>/images/your_image.png') no-repeat;
padding-left: 50px;
}