Hi Keir,
Yes, you will need to seal the image first. If you are ever unsure, you need to ask yourself whether getting the item wet would distort it in any way. If the answer is yes, then you need to seal.
On another note, I have found that vintage photographs are usually printed on acid containing paper and do not do well in resin. I would suggest considering scanning the image and reprinting onto a new piece of white paper before sealing and including in the resin.