Adding a custom post type icon from the CPT icon set. Icons and code from Randy Jensen: http://randyjensenonline.com/thoughts/wordpress-custom-post-type-fugue-icons/ WordCamp San Diego 2013

cpt-icons.php

<?php
/*

    Add custom icon with grayscale to color activation states.
    Makes you more like core.
    
    We're assuming this is included in your core plugin file
    and the icon cpt-icon.png is in the same directory.
    
    So, if this file were /wp-content/plugins/my-plugin/plugin.php
    the icon file would be /wp-content/plugins/my-plugin/cpt-icon.png
    
    Be sure to check out these great icons and original source:
    http://randyjensenonline.com/thoughts/wordpress-custom-post-type-fugue-icons/
    
*/
add_action( 'admin_head', 'szbl_cpt_icons' );
function szbl_cpt_icons()
{
    $post_type = 'szbl-post-type-slug';
?>
<style type="text/css" media="screen">
#menu-posts-<?php echo $post_type; ?> .wp-menu-image
{
    background: url(<?php echo plugins_url( '/cpt-icon.png', __FILE__ ); ?>)
        no-repeat 6px -17px !important;
}
#menu-posts-<?php echo $post_type; ?>:hover .wp-menu-image,
#menu-posts-<?php echo $post_type; ?>.wp-has-current-submenu .wp-menu-image
{
        background-position:6px 7px!important;
}
</style>
<?php
}