Photojojo.com has a neat idea on how to combine photos, both old and new. Their article shows you how to combine a vintage/antique photo and a specific location today - it's simple and creates a very neat effect. You'll be able to overlap the timeline of your ancestor's lives and your own. Check out some examples.
But many people say: I don't have any old photos of my ancestor's! That's ok, Flickr has the solution. Like the original photographer, take advantage of The Commons on Flickr. This is a collection of photographs from museums and archives from around the world - and they are copyright free!