I don't have time to read all the previous responses, so forgive me for any duplication. Many people would be surprised to learn that they own no rights to their family photos--unless they took them, themselves, or are the copyright heirs of those who did. We are so accustomed to institutions that house old photos acting like they own the rights to them and can tell anyone exactly what they can and cannot do with them that we think we are in the same position--because the photos are in our custody. Yet there is a vast difference between stewardship [housing and caring for the photos] and owning any rights to them. As with any other creative effort, the rights to the photos belong only to the photographers--unless they entered into a specific legal agreement that the photos were a work for hire. In the US and in England, the copyright to anything lasts for the life of the creator plus 70 years. That means, within those 70 years, the photographer might have a copyright heir that now controls it. But, after those 70 years have lapsed--that's it, finito, done. The work enters into the Public Domain.
Unless you can truthfully say that you have never, ever, used something that was in the Public Domain, whether a photograph or an ebook--don't make an exception for yourself and your family photos. If they are from the 19th Century, they are in the Public Domain and you can't prevent anyone from appropriating them. You own them, they are in your care--but you have no rights over them whatsoever. So share them freely and for God's sake don't spoil the creation and the view of the creator by putting an ugly watermark or something on them. They are somebody's best work, the result of their talent and expertise and you really don't have the right to change them in any way. It's no different from writing your name or logo--whatever--on a painting in a museum. Think about it. But, if it really bothers you so much to share the old photos, then take them offline or don't put them on in the first place. You can't have it both ways.