There isn’t a filter because the call is part of the theme code, not WP. This should help get you started:
1. Create a folder in your child theme: /img
2. Add your image to that folder and name it “thumb-medium.png”
3. Copy the content.php file from the parent theme to your child theme.
4. Find this block of code near the top of content.php:
<?php if ( has_post_thumbnail() ): ?>
<?php the_post_thumbnail('thumb-medium'); ?>
<?php elseif ( ot_get_option('placeholder') != 'off' ): ?>
<img src="<?php echo get_template_directory_uri(); ?>/img/thumb-medium.png" alt="<?php the_title(); ?>" />
<?php endif; ?>
Change the <img> tag to get the file from your child theme:
<img src="<?php echo get_stylesheet_directory_uri(); ?>/img/thumb-medium.png" alt="<?php the_title();
You can do the same thing with content-featured and content-standard, although content-standard uses “thumb-standard.png”. For the sidebar widgets, alx-posts.php uses “thumb-medium.png” and alx-tabs.php uses “thumb-small.png”.