Add your categories to WordPress.com's sitemap.xml.

wpcom-sitemap-categories.php

<?php
/**
 * Adds categories to the sitemap by filtering wpcom_print_sitemap.
 *
 * @uses array_key_exists, esc_url, function_exists, get_categories, is_array, taxonomy_image_plugin_get_associations, taxonomy_image_plugin_get_image_src, trailingslashit, wpcom_sitemap_array_to_simplexml
 * @return array
 */
function nesn_filter_wpcom_print_sitemap( $tree, $latest_mod ) {
  $url_base = trailingslashit( get_bloginfo( 'siteurl' ) );
	$categories =  array();
	$category_images = null;
	$url = null;

	// Sanity check. This function will only be available if the
	// taxonomy image plugin is enabled.
	if ( function_exists( 'taxonomy_image_plugin_get_associations' ) ) {
		$category_images = &taxonomy_image_plugin_get_associations();
	}

	// Iterate over each category
	foreach ( get_categories() as $category ) {
		$url = array(
			'loc' => esc_url( $url_base . $category->slug ),
			'changefreq' => 'monthly',
			'image' => null
			);

		// More sanity checks
		if ( is_array( $category_images ) && function_exists( 'taxonomy_image_plugin_get_image_src' ) ) {
			// Does the category have an image associated with it?
			if ( array_key_exists( $category->term_id, $category_images ) ) {
				$url['image']['loc'] = esc_url( taxonomy_image_plugin_get_image_src( $category_images[ $category->term_id ] ) );
			}
		}

		// Wrap the entire URL object in an array so that it is properly
		// formatted when the XML is printed to stdout.
		wpcom_sitemap_array_to_simplexml( array( 'url' => $url ), $tree );
	}

	return $tree;
}

add_filter( 'wpcom_print_sitemap', 'nesn_filter_wpcom_print_sitemap', 10, 2 );