Give this a try and see how it works:
1. Copy index.php from the parent theme to your child theme.
2. At the top of the file, insert a new <div> for your box between “featured” and “have_posts”. The <p> element will be used to overlay the text on the box image. If your image already has the text you can omit that.
<?php get_template_part('inc/featured'); ?>
<div class="my-recent-posts-box">
<p>Recent Posts</p>
</div>
<?php if ( have_posts() ) : ?>
3. Add some css to set the box background image and style the heading:
/* custom heading box */
.my-recent-posts-box {
background-image: url("https://path_to_your_image.jpg");
background-repeat: no-repeat;
background-color: #00f; /* blue */
width: 100%;
height: auto;
margin: 20px 0;
}
.my-recent-posts-box p {
color: #fff;
font-weight: bold;
margin-left: 20px;
}
You can combine the three background properties into one:
background: #00f url("http:/path_to_your_image.jpg") no-repeat;
Note: the box background color is there only as a backup in case the image doesn’t display for some reason.