Posts page not showing correct image

If the theme you’re using is this: https://wordpress.org/themes/simple-business-wp/, then… This is the code responsible for images on the blog page:

<?php
if (has_post_thumbnail()) {
    the_post_thumbnail('nimbus_722_400', array('class' => 'nimbus_722_400 img-responsive'));
} else { ?>
    <?php if ( (simple_business_wp_get_option('fp-news-thumbs-toggle') == "1") || (simple_business_wp_get_option('fp-news-thumbs-toggle') == "") ) { ?>
        <img src="https://wordpress.stackexchange.com/questions/330187/<?php echo get_template_directory_uri(); ?>/assets/images/preview/722x400-<?php echo rand(1,8); ?>.jpeg" class="nimbus_722_400 img-responsive" alt="<?php the_title(); ?>" />
    <?php } ?>
<?php } ?>

As you can see, it will show post thumbnail, if it is set. And if there is no thumbnail set and some option is set to true, then it will show random image from assets.

So if you’re seeing some random images on the blog list, then probably you haven’t set featured images for these posts.