Hi @atise
If you’re creating an image field, when you add the code to your template (please see here), you can add the class to your img tag.
Once you add the class, say ‘responsive-img’, you can then reference that in your CSS.
So:
<?php
$image = get_field('image');
if( !empty( $image ) ): ?>
<img src="<?php echo esc_url($image['url']); ?>" alt="<?php echo esc_attr($image['alt']); ?>" class="responsive-img" />
<?php endif; ?>
Then in your CSS:
.responsive-img {
/*whatever you need to add*/
}