<style type="text/css">
#toplevel_page_wordpress-related-posts .wp-menu-image {
background: url('<?php echo $menu_icon; ?>') 7px 6px no-repeat;
}
@media only screen and (-webkit-min-device-pixel-ratio: 1.5) {
#toplevel_page_wordpress-related-posts .wp-menu-image {
background-image: url('<?php echo $menu_icon_retina; ?>');
background-size: 16px 16px;
}
}
</style>