using other image instead of post thumbnail

Hi there,

My theme has a featured image built in. The image is displayed using this code:

<a href="<?php the_permalink(); ?>">? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?if (get_post_meta($post->ID, 'blog-full-image', true)){
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??<img class="extended-blog-image" src="<?php echo get_post_meta($post->ID, 'blog-full-image', true); ?>" alt="<?php the_title(); ?>" />
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??</a>

I’m trying to make YARPP show that image (blog-full-image) instead of the post-thumbnail. I know this isn’t how the plugin is designed to work, but it would make my life a lot easier if I could make this happen. Could anyone help?

ps – i know just enough code to get myself into trouble, so please forgive any ignorance in this question.
pps- here’s my site: https://sparkflyphotography.com

https://www.ads-software.com/plugins/yet-another-related-posts-plugin/