How to add “Read More” button

– do you want this to happen only for the mentioned category archive?
– do you want this to happen only for ‘image’ postformats?

generally, for all archive pages, and only for ‘image’ postformats, in a child theme of Twenty Seventeen, possibly create and edit content-image.php in the folder /template-parts/post/,
and after this section:

		<div class="post-thumbnail">
			<a href="<?php the_permalink(); ?>">
				<?php the_post_thumbnail( 'twentyseventeen-featured-image' ); ?>
			</a>
		</div><!-- .post-thumbnail -->

insert this kind of code:

		<div class="read-more">
			<a href="<?php the_permalink(); ?>">
				Read More
			</a>
		</div>

and add for example this CSS:

.format-image .read-more {
	display: block;
	text-align:center;
	width: 140px;
	padding: 10px;
	background: #000; 
	margin: auto;
}
.format-image .read-more a {
	color: #fff;
	text-transform: uppercase;	
}