<?php

//add custom image size here if required in future.
function acclivity_register_custom_image_sizes() {
    $img_slug = 'acclivity_image_slug';
    $width = 350;
    $height = 350;
    $crop = true;
    add_image_size( $img_slug, $width, $height, $crop ); 
}

function acclivity_add_custom_image_sizes($sizes) {
    return array_merge($sizes, [
        'acclivity_image_slug' => __('New Image Size', 'acclivity'),
    ]);
}