“New” in latest post

I am using this code to get a “new” gif in the latest code:
CSS=
\.firstpost {
/*background-color: #2E3233;*/
padding: 10px;

background-image: url(‘https://domain.tld/images/latest.gif’);
background-repeat: no-repeat;
background-position: right bottom
}
.entry{
padding: 10px;
}\
And the code i am using in index.php=
\<p><div class=”entry <?php if (!$notFirstPost){echo ‘firstpost’;} ?>”></div></p>
<?php $notFirstPost = true; ?>\

This worked fine until i had to do a “sticky” post. Now, that “sticky” post has the “new” gif in it. So, it appears to me that i need to change the code from “firstpost”, somehow, to latest post. How do i get the “new” gif to appear in the latest post? Look here