Put a class on every image

Not all Images are inserted in Enfold as <img src=" …
a lot of them are only background-images f.e.: https://kriesi.at/themes/enfold-2017/elements/masonry/

you can try this in child-theme functions.php if you need that class directly on the img tag itself
if you need it on the parent element tell me – then we can find that parent by another selector.
( i think even an each function is not neccessary)

function add_class_to_images(){
?>
<script>
(function($) {		
	$('img').addClass('zoooom');
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'add_class_to_images');

Well you can use every conditional tag wordpress offers: https://codex.wordpress.org/Conditional_Tags
a douple pipe is a logic or and a double && is a logic and
so you can start with – the if clause wraps the script that should be used.
e.g.:

function add_class_to_images(){
	if(is_page() || is_singular('post') || is_singular('portfolio')){
		?>
			<script>
				(function($) {		
					$('img').addClass('zoooom');
				})(jQuery);
			</script>
		<?php
	}   // this closes the if statement
}   // this closes the function itself
add_action('wp_footer', 'add_class_to_images');

//more specific selections can be made within the brackets. (e.g. - which pages should be affected via an array). 
// IDs without quotes and page names like privacy , portfolio etc. with quotes
if(is_page(array( 1396, 1617, 'privacy'))){ …

it might be better to see your pages it concerns to give exact help on that.

sorry no private content for me – i’m participant as you!

but all pages except the front page :

function add_class_to_images(){
if(is_page() && !is_front_page()){
?>
<script>
	(function($) {		
		$('img').addClass('zoooom');
	})(jQuery);
</script>
<?php
}
} 
add_action('wp_footer', 'add_class_to_images');

the Exclamation mark indicates a not logic so it is all pages but not front page
PS: is_home() might work here too – but there is a little difference inbetween is_front_page() and is_home()