Males: consider suggesting your male employees wear a shirt and tie without loud patterns or stripes. They should also bring with them at least a sports coat. Headshots do not look nice if men only wear white shirts and ties in their shots. The white shirts can easily blend in with the white background and make men look like floating heads.
Women: consider suggesting the women in your office wear blouses and blazers. As long as there are no loud prints, and they are not wearing white, they should look wonderful.