Not entirely ethical. Depending if the unit has changed since 2013, you may be violating article 12 of the code of ethics. Also, there comes the legality on who the pictures belong to. With some real estate boards, when the pictures are used for a MLS listing, they become property of the MLS board, and not the agent who took the pictures. Finally, with some listings, you can look at when it was listed previously in the MLS. Other agents are going to see that you reused pictures from the previous listing and that may reflect badly on you. If it were me, I'd explain this to my clients and just take new pictures.