Adds image field in category description.

category-image-field.php

<?php
/*
 * Plugin Name: Category Image Field
 * Plugin URI: http://claudiosmweb.com/
 * Description: Adds image field in category description.
 * Version: 0.1
 * Author: Claudio Sanches
 * Author URI: http://claudiosmweb.com/
 * License: GPLv2 or later
 */

class Category_Image_Field {

    public function __construct() {
        add_action( 'edit_category_form_fields', array( $this, 'register_fields' ) );
        add_action( 'edited_category', array( $this, 'save_fileds' ) );

        if ( isset( $_GET['taxonomy'] ) && $_GET['taxonomy'] == 'category' ) {
            add_action( 'admin_init', array( &$this, 'scripts' ) );
        }
    }

    /**
     * Load options scripts.
     *
     * @return void
     */
    function scripts() {
        wp_enqueue_script( 'jquery' );
        wp_enqueue_script( 'media-upload' );
        wp_enqueue_script( 'thickbox' );
        wp_enqueue_style( 'thickbox' );
    }

    /**
     * Register fields.
     *
     * @param  object $term  Object with term data.
     *
     * @return string        Form new fields.
     */
    public function register_fields( $term ) {
        $category_meta = get_option( 'category_' . $term->term_id );
        $default_image = $category_meta['image'] ? $category_meta['image'] : '';
        $default_url = $category_meta['url'] ? $category_meta['url'] : '';

        $html = '<tr>';
            $html .= '<th valign="top" scope="row">';
                $html .= sprintf( '<label for="category-meta-image">%s</label>', __( 'Imagem', 'cif' ) );
            $html .= '</th>';
            $html .= '<td>';
                $html .= sprintf( '<input type="text" id="category-meta-image" name="category_meta[image]" value="%s" class="regular-text" />', $default_image );

                $html .= sprintf( ' <input class="button" id="category-meta-image-button" type="button" value="%s" />', __( 'Selecionar arquivo', 'cif' ) );

                $html .= '<script type="text/javascript">';
                    $html .= 'jQuery(document).ready(function($) {';
                        $html .= '$("#category-meta-image-button").click(function() {';
                            $html .= 'uploadID = $(this).prev("input");';
                            $html .= 'formfield = $("category-meta-image").attr("name");';
                            $html .= 'tb_show("", "media-upload.php?post_id=&amp;type=image&amp;TB_iframe=true");';
                            $html .= 'return false;';
                        $html .= '});';
                        $html .= 'window.send_to_editor = function(html) {';
                            $html .= 'imgurl = $("img", html).attr("src");';
                            $html .= 'uploadID.val(imgurl);';
                            $html .= 'tb_remove();';
                        $html .= '}';
                    $html .= '});';
                $html .= '</script>';

            $html .= '</td>';
        $html .= '</tr>';

        $html .= '<tr>';
            $html .= '<th valign="top" scope="row">';
                $html .= sprintf( '<label for="category-meta-url">%s</label>', __( 'URL', 'cif' ) );
            $html .= '</th>';
            $html .= '<td>';
                $html .= sprintf( '<input type="text" id="category-meta-url" name="category_meta[url]" value="%s" class="regular-text" />', $default_url );
            $html .= '</td>';
        $html .= '</tr>';

        echo $html;
    }

    /**
     * Save category custom fields.
     *
     * @param  int $term_id  Term ID.
     *
     * @return update term fields.
     */
    public function save_fileds( $term_id ) {
        if ( isset( $_POST['category_meta'] ) ) {
            $category_meta = get_option( 'category_' . $term_id );
            $keys = array_keys( $_POST['category_meta'] );

            foreach ( $keys as $key ) {
                if ( isset( $_POST['category_meta'][$key] ) ) {
                    $category_meta[$key] = $_POST['category_meta'][$key];
                }
            }

            // Save the option array.
            update_option(  'category_' . $term_id , $category_meta );
        }
    }
} // close Category_Image_Field class.

$cs_category_image_field = new Category_Image_Field;

/**
 * Get category image URL.
 *
 * @return string Image URL.
 */
function get_cat_image_url() {
    // Get the current category ID.
    $term_id = get_query_var( 'cat' );
    // Get options.
    $meta = get_option( 'category_' . $term_id );

    if ( isset( $meta['image'] ) ) {
        return esc_url( $meta['image'] );
    }
}

/**
 * Get category meta url.
 *
 * @return string Meta URL.
 */
function get_cat_url() {
    // Get the current category ID.
    $term_id = get_query_var( 'cat' );
    // Get options.
    $meta = get_option( 'category_' . $term_id );

    if ( isset( $meta['url'] ) ) {
        return esc_url( $meta['url'] );
    }
}

/**
 * Get category image HTML.
 *
 * @return string Image HTML.
 */
function get_cat_image() {
    // Get the current category ID.
    $term_id = get_query_var( 'cat' );
    // Get options.
    $meta = get_option( 'category_' . $term_id );

    if ( isset( $meta['image'] ) ) {
        return sprintf( '<a href="%3$s" title="%2$s"><img src="%1$s" alt="%2$s" /></a>', esc_url( $meta['image'] ), single_cat_title( '', false ), esc_url( $meta['url'] ) );
    }
}