From the title of the question, I believe the solution would be:
div > p > img {
padding: 10px;
}
The operator > will fetch any direct child element among the related elements. This is, div > p search for all elements p which have a parent element div and p > img search for all elements img which have a parent element p.
Take an example:
div > p > img {
padding: 10px;
border: 1px solid black;
}
<div class="box1">
<p>texto</p>
<p>texto</p>
<p><img src="http://via.placeholder.com/350x150"></p>
<p>texto</p>
</div>
<div class="box2">
<p>texto</p>
<p>texto</p>
<p><img src="http://via.placeholder.com/350x150"></p>
<p>texto</p>
</div>
How >, + , ~ selectors work in CSS?