Så här ställer du in en utvald image baserad på kategorin för inlägg i WordPress

/**
 * Plugin Name: Category Thumbnail Fallback
 * Description: Use the category image as fallback when the post does not have a featured image
 */

class WPBCategoryThumbnailFallback
{
    protected static $taxonomies = ['category'];

    protected $nonceId = 'wpb_category_thumb_fallback_none';
    protected $fieldId = 'wpb_category_fallback_post_image';

    public $taxonomy;

    protected function __construct($taxonomy)
    {
        $this->taxonomy = $taxonomy;
    }

    public static function init()
    {
        foreach (static::$taxonomies as $taxonomy) {
            $_self = new self($taxonomy);

            add_action('admin_enqueue_scripts', [$_self, 'scripts']);

            add_action("{$taxonomy}_add_form_fields", [$_self, 'add']);
            add_action("{$taxonomy}_edit_form_fields", [$_self, 'edit'], 99, 2);

            add_action("created_{$taxonomy}", [$_self, 'saveTerm'], 10, 2);
            add_action("edited_{$taxonomy}", [$_self, 'editTerm'], 10, 2);

            add_filter("get_post_metadata", [$_self, 'fallback'], 99, 5);
        }
    }

    public function scripts($hook_suffix)
    {
        if (in_array($hook_suffix, ['term.php', 'edit-tags.php'])) {
            $screen = get_current_screen();

            if (is_object($screen) && "edit-{$this->taxonomy}" == $screen->id) {
                wp_enqueue_media();

                wp_add_inline_script('media-editor', $this->inlineScript());
            }
        }
    }

    public function add()
    {
        ?>
        <div class="form-field upload_image-wrap">
            <label for="upload_image">Image</label>
            <input id="upload_image" type="hidden" size="36" name="<?php
            echo esc_attr($this->fieldId)
            ?>" value=""/>
            <div id="wpb-category-image-preview" style="max-width: 150px; max-height: 150px;"></div>
            <input id="upload_image_button" class="button" type="button" value="Upload Image"/>
            <p>Enter a URL or upload an image</p>
        </div>
        <?php
        wp_nonce_field($this->nonceId, $this->nonceId);
    }

    public function edit($term, $taxonomy)
    {
        $value = get_term_meta($term->term_id, $this->fieldId, true);
        $image = wp_get_attachment_image((int)$value);
        ?>
        <tr class="form-field upload_image-wrap">
            <th scope="row"><label for="name">Image</label></th>
            <td>
                <label for="upload_image">
                    <input id="upload_image" type="hidden" size="36" name="<?php
                    echo esc_attr($this->fieldId)
                    ?>" value="<?php
                    echo esc_attr($value)
                    ?>"/>
                    <div id="wpb-category-image-preview" style="max-width: 150px; max-height: 150px;"><?php
                        echo $image;
                        ?></div>
                    <input id="upload_image_button" class="button" type="button" value="Upload Image"/>
                </label>
                <p class="description">Enter a URL or upload an image</p>
            </td>
        </tr>
        <?php
        wp_nonce_field($this->nonceId, $this->nonceId);
    }

    public function saveTerm($term_id, $tt_id)
    {
        $data = array_filter(wp_unslash($_POST), function ($value, $key) {
            return in_array($key, [$this->nonceId, $this->fieldId]);
        }, ARRAY_FILTER_USE_BOTH);

        if (
            empty($data) ||
            empty($data[$this->nonceId]) ||
            ! wp_verify_nonce($data[$this->nonceId], $this->nonceId) ||
            ! current_user_can('manage_categories')
        ) {
            return null;
        }

        if (empty($data[$this->fieldId]) || empty(absint($data[$this->fieldId]))) {
            return delete_term_meta($term_id, $this->fieldId);
        }

        $value = absint($data[$this->fieldId]);

        return update_term_meta($term_id, $this->fieldId, $value);
    }

    public function editTerm($term_id, $tt_id)
    {
        $this->saveTerm($term_id, $tt_id);
    }

    public function fallback($null, $object_id, $meta_key, $single, $meta_type)
    {
        if (
            $null === null &&
            $meta_key === '_thumbnail_id'
        ) {
            $meta_cache = wp_cache_get($object_id, $meta_type . '_meta');

            if ( ! $meta_cache) {
                $meta_cache = update_meta_cache($meta_type, [$object_id]);
                $meta_cache = $meta_cache[$object_id] ?? null;
            }

            $val = null;

            if (isset($meta_cache[$meta_key])) {
                if ($single) {
                    $val = maybe_unserialize($meta_cache[$meta_key][0]);
                } else {
                    $val = array_map('maybe_unserialize', $meta_cache[$meta_key]);
                }
            }

            if (empty($val)) {
                $fallbackImageId = $this->getPostFallbackImageId($object_id, $single);

                if ( ! empty($fallbackImageId)) {
                    return $fallbackImageId;
                }
            }

            return $val;
        }

        return $null;
    }

    public function getPostFallbackImageId($postId, $single)
    {
        $terms = get_the_terms($postId, $this->taxonomy);

        if (empty($terms) || is_wp_error($terms)) {
            return null;
        }

        foreach ($terms as $term) {
            $fallbackIdFromCategoryId = get_term_meta($term->term_id, $this->fieldId, $single);

            if ( ! empty($fallbackIdFromCategoryId)) {
                return $fallbackIdFromCategoryId;
            }
        }

        return null;
    }

    public function inlineScript()
    {
        return "jQuery(document).ready(function ($) {
            var custom_uploader;
            $('#upload_image_button').click(function (e) {
                e.preventDefault();
                //If the uploader object has already been created, reopen the dialog
                if (custom_uploader) {
                    custom_uploader.open();
                    return;
                }
                //Extend the wp.media object
                custom_uploader = wp.media.frames.file_frame = wp.media({
                    title: 'Choose Image',
                    button: {
                        text: 'Choose Image'
                    },
                    multiple: true
                });
                //When a file is selected, grab the URL and set it as the text field's value
                custom_uploader.on('select', function () {
                    console.log(custom_uploader.state().get('selection').toJSON());
                    attachment = custom_uploader.state().get('selection').first().toJSON();
                    
                    var thumbUrl = attachment && attachment.sizes && attachment.sizes.thumbnail && attachment.sizes.thumbnail.url
                    ? attachment.sizes.thumbnail.url
                    : attachment.url;
                    $('#wpb-category-image-preview').html('<img src=\"'+ thumbUrl +'\">');
                    $('#upload_image').val(attachment.id);
                });
                //Open the uploader dialog
                custom_uploader.open();
            });
        });";
    }
}

protected static $taxonomies = ['category'];