Yes, this is possible.
Find in index.php
$sql = "SELECT i.image_id, i.cat_id, i.user_id, i.image_name, i.image_description, i.image_keywords, i.image_date, i.image_active, i.image_media_file, i.image_thumb_file, i.image_download_url, i.image_allow_comments, i.image_comments, i.image_downloads, i.image_votes, i.image_rating, i.image_hits".$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 NOT IN (".get_auth_cat_sql("auth_viewcat", "NOTIN").")
ORDER BY i.image_date DESC
LIMIT $num_new_images";
Replace this with following
$new_cutoff = time() - 60 * 60 * 24 * $config['new_cutoff'];
$sql = "SELECT IF(i.image_date >= $new_cutoff,RAND()+1,1) as random_no, i.image_id, i.cat_id, i.user_id, i.image_name, i.image_description, i.image_keywords, i.image_date, i.image_active, i.image_media_file, i.image_thumb_file, i.image_download_url, i.image_allow_comments, i.image_comments, i.image_downloads, i.image_votes, i.image_rating, i.image_hits".$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").")
ORDER BY random_no DESC, i.image_date DESC
LIMIT $num_new_images";
It will randomize the new wallpapers ..
Thanks,