Need to add class=”lightbox” for every single post image

One way you can accomplish it is this way:

functions.php

add_action( 'wp_enqueue_scripts', 'your_lightbox_callback' );
function your_lightbox_callback(){

    if( is_single() /* or whatever here */ ) {
        wp_register_script( 'your-lightbox', get_bloginfo( 'template_url' ) . '/lightbox-class.js' );
        wp_enqueue_script( 'your-lightbox' );
    }

}

lightbox-class.js

var $j = jQuery.noConflict();
$j( document ).ready( function() {

    $j( '#whereveryourpostsare img' ).addClass( 'lightbox' );

} );

Let me add that this hasn’t been tested, just typed here on the fly. If you try it do tell me if you have any issues, thanks!