How to add icon over specific thumb

The implementation can be purely CSS, and depends on your specific HTML markup. However, the class you’ll need to target is supplied by the post_class() template tag.

Let’s say you have something like the following, basic markup:

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

    <div class="post-title">

        <img ... />
        <h1><?php the_title(); ?></h1>

    </div>

    <!-- Etc. -->

</div>

The post_class() template tag will output a category-based class, category-{name}, and also, category-{id}. So, you can target either one in your stylesheet:

.category-{name} .post-title img {}

or

.category-{id} .post-title img {}

It’ll be specific to your markup, of course; but hopefully you get the idea.