Use thumbnail attachment instead of avatar on search results page

I have duplicated the template files in my own theme and am working through styling the searvh results pages. My question is:

Is there a way to bring in the thumbnail for the post in the results instead of the authors avatar? I have tried adding via the normal WordPress route but no image shows.

<a href="<?php echo network_get_permalink() ?>"><?php the_post_thumbnail('sliderbox3', array 'class' => "img-responsive attachment-post-thumbnail")); ?>(</a>

Any ideas?