I tried this with a photo of my grandmother and a photo of of the man I now believe to be her father, thanks to DNA testing. That match comes up at 30%, I then compared Nan with a photo said to be of his mother and that came out at 96%! I wonder if his moustache skewed the results at all
I'll have to do as Merry suggests and compare two photos where I know the relationship to see how the results pan out.
Great fun, though thanks Tom highlighting it!
edit: Just compared my aunt to the same photo of her mother, my grandmother and that came up at 88%. Very interesting!