Add code to the Loop to display multiple images

I have to add code to the WordPress-loop, because I want to display multiple images when visitors submit posts with the ‘User Submitted Posts’ plugin. I am using the SUVTown-Theme. To which file should I add this code and where?

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<?php // USP display linked images
$args = array('order' => 'ASC','post_type'=>'attachment','post_parent'=>$post->ID,'post_mime_type'=>'image','post_status'=>null,'numberposts'=>-1,);
$attachments = get_posts($args);
if ($attachments) {
	foreach ($attachments as $attachment) {
		echo '<a rel="lightbox[gallery]">ID).'" title="'.get_the_title().'">'.wp_get_attachment_image($attachment->ID,'medium',false).'</a>';
	}
} ?>

I have already tried to add it to the Index.php file, but it didn’t work. Does someone have a solution?