Easiest way to do this would be in a content-type template. You want to create a template for your content type, ex. node--[type|nodeid].tpl.php and add this in your theme folder. In your template, for displaying the boolean field you could use logic like this:

    <?php 
    if ($node->field_boolean_field[$node->language][0]['safe_value'] == 0) { 
        print "<img src='no_img.jpg'>";
    } else {
        print "<img src='yes_img.jpg'>";
    }
    ?>

This similar question may help as well:

http://drupal.stackexchange.com/questions/19133/optionally-add-image-icon-after-node-title-based-on-boolean-content-type-field