how to add icon category every post category ?

The most simple way of doing this is by checking if a thumbnail exists, and if so, checking that a category is assigned, and if so, adding the icon.

In the Page Template, you’d have something like this:

<div class="thumbnailContainer">
  <?php
    if ( has_post_thumbnail() ) {
      the_post_thumbnail();
      $singleCatTitle = single_cat_title("", false);
      if(single_cat_title("", false)){
  ?>
  <img class="thumbnailBannerIcon" src="/path/to/<?php echo $singleCatTitle ?>.png"/>
  <?php
      }
    }
  ?>
</div>

Then, by using “position: relative;” on the thumbnail container, and “position: absolute;” on the icon, you can place it in the corner, appropriately.