I start using photo subtitles more and more as I increasingly move away from staged/fake stock images and use real images in my presentations. When using a creative common image from Flickr, it is important to give credit to the photographer, that is one thing to in the footer.
But the photo footer can also include a little bit more background information on what we see in the image, information that does not always have to contribute to the slide. The full details of the painter, the painting title and the place where the painting is currently displayed. The fact that the Paris cafe you see on the image is actually Cafe de Flore, in an image from 2006.
The Big Picture section of boston.com should feature in the RSS reader of every presentation designer. It is an almost daily stream of beautiful images (often more than 1MB a piece). The image below (related to the Diwali celebrations) was taken from it. You see a good way to format an image subtitle (with - in this case a lot of - information) as white text in black at the bottom of the photo.