How to use thumbnail size of image if I’m only using src to get image

I have a custom field for an image, and using it this way:

<?php $item_img = get_sub_field('image'); ?>
<img src="<?php echo $item_img ?>" />

But I want to use the thumbnail version of the image, as set to 150×150 in wordpress.

How do I do that with this type of setup?

Do I have to set a custom add_image_size() or does wordpress have duplicated thumbmail size images?