Need to show only one image in the lightbox

best would be to see your site :
but if i understand it right : you have the images not as image alb element but you have it in text-block alb inserted different images?

so if you only wants to get rid of gallery function for those images and on a specific page only – you need your page-id
here in my example code it is the : 5

put the code ( change the page id) to your child-theme functions.php:

function popup_no_gallery() { 
if( is_page(5) ) {
?>
<script type="text/javascript">
(function($){
$(window).load(function(){
    $('a.lightbox-added').magnificPopup({ 
		type:  'image',
		mainClass: 'avia-popup mfp-zoom-in mfp-image-loaded anchor-link',
		preloader: true,
		midClick: true,

		gallery: {
			enabled: false
		},	
	}); 
});
})(jQuery);
</script>
<?php 
}
}
add_action('wp_footer', 'popup_no_gallery');