try this:
$cat_in = array(2,7,8,11,12,15,16,18); //list of category ids // *** TO FILTER RANDOM IMAGES ***
$sql = "SELECT i.image_id, i.cat_id, i.user_id, i.image_name, i.image_description, i.image_keywords, i.image_date, i.image_active, i.image_media_file, i.image_thumb_file, i.image_download_url, i.image_allow_comments, i.image_comments, i.image_downloads, i.image_votes, i.image_rating, i.image_hits".$additional_sql.", c.cat_name".get_user_table_field(", u.", "user_name")."
FROM ".IMAGES_TABLE." i, ".CATEGORIES_TABLE." c
LEFT JOIN ".USERS_TABLE." u ON (".get_user_table_field("u.", "user_id")." = i.user_id)
WHERE i.image_active = 1 AND c.cat_id = i.cat_id AND i.cat_id IN (".get_auth_cat_sql("auth_viewcat").") AND i.cat_id IN (".implode(", ", $cat_in).")
ORDER BY RAND()
LIMIT $num_new_images";