Twenty Seventeen theme: custom left part under the page title

There doesn’t seem to be any specific function to hook to, so a back-end solution would require altering the “/template-parts/page/content-page.php template file (via a child theme, preferably), namely adding your desired elements right before the closing </header> tag, for example:

<header class="entry-header">
    <?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
    <?php twentyseventeen_edit_link( get_the_ID() ); ?>
    <!-- START new code -->
    <img src="http://image-source.jpg">
    <span class="left-text">Insert your text here</span>
    <!-- END new code -->
</header><!-- .entry-header -->

You can also add conditional statements if you wish the new content to be displayed on a specific page/pages, e.g. for a front page:

<!-- START new code -->
<?php if ( is_front_page() ) { ?>
    <img src="http://image-source.jpg">
    <span class="left-text">Insert your text here</span>
<?php } ?>
<!-- END new code -->

for a specific page ID:

<!-- START new code -->
<?php if ( 25 == get_the_ID() ) { ?>
    <img src="http://image-source.jpg">
    <span class="left-text">Insert your text here</span>
<?php } ?>
<!-- END new code -->