I received it over the weekend. It's the first time that either I or my dad have ever seen a picture of him.
The reason for this is that he left his wife (my grandfather's mother) for another woman when my grandfather was about 14. The topic of my grandfather's father was thenceforth declared taboo. Neither my grandfather nor his sister would talk about it when I asked, nor would they even say why they wouldn't talk about it. I only found out what happened years later when one of the grandchildren of my great-grandfather by his second wife (I say wife, but we're not actually sure they married) got in touch. It's from this Australian half-cousin of my dad's that the above photo came. She doesn't have any more herself, but is hoping her sister might have one or two.
Somewhere out there are photos of my grandfather's paternal grandfather. I know they exist, because my grandfather had some; sadly, they were lost when he died and his second wife (he remarried after my grandmother died) threw them all out. However, given that my grandfather's grandfather had 16 children who made it to adulthood, I hold the not-unreasonable hope that some distant cousin has a stash of them somewhere. My grandfather showed me one once when I was aged maybe 10 or so, so it's possible I could even recognise his grandfather from a photo that didn't have a name on it. I'm not sure, though: I'd know the same photo, but not necessarily the same man in a different photo.
There is something of a family resemblance between my grandfather and his father. This shouldn't be surprising, but strangely I wasn't actually expecting it.