Add custom version to an image

so you can do that in 2 different ways
1. modify directly on template-parts/content-single.php
2. you can plug it using the

add_action(pre_get_post,func_with_your_staff ) 

inside this

    function func_with_your_staff( $query ) {
        global $wp ; 
     if ( $query->is_single() && empty($query->post_type) $query->set('post_type', 'wherever_is_type'); // perhaps you have a custom_post_type 
    ?> and then you can add your extra staff <img src"" class="big-img"> <?
} 

3. you can use jQuery(myImg).css(‘width’,’600px’)