I believe that woman who is his aunt, that lives in Arkansas, said he is the spitting image of her brother, albeit dark. If his own relatives say he looks exactly like Stanley, I'd say that seems to be the consensus of most people as well. I can find the link for you if you want, but you can probably google-fu it as well as I can.
I am with you in that I believe the naked pictures are of Ann Dunham and that Frank took them.
Here are some excerpts with Toot’s thoughts on being a Granny :
1. ‘“Quite sensibly,” Toot says from the kitchen. (We call my grandmother Tutu, Toot for short; it means “grandparent” in Hawaiian, for she decided on the day I was born that she was still too young to be called Granny.) Gramps scowls but decides to ignore her.’ (Dreams from My Father Ch 1)
2. Madelyn Dunham, who was ambitiously pursuing her career as a bank executive, did not want to think of herself as a grandmother. She preferred to be called Toot by Barry, so Toot it was…Even when he was living in Indonesia, Barry had spent summers with Gramps and Toot. (The Dream Begins How Hawai’I shaped Barack Obama by Stu Glauberman & Jerry Burns Ch 6 pg 57)
3. ‘I was intrigued by old Frank, with his books and whiskey breath and the hint of hard-earned knowledge behind the hooded eyes. The visits to his house always left me feeling vaguely uncomfortable, though, as if I were witnessing some complicated, unspoken transaction between the two men, a transaction I couldn’t fully understand. The same thing I felt whenever Gramps took me downtown to one of his favorite bars, in Honolulu’s red-light district.
“Don’t tell your grandmother, [Madelyn Payne]” he would say with a wink,’ (Dreams from My Father Ch 4)
NOTE: - This Dreams excerpt seems to imply there is some sort of inside grandmother joke between Frank Marshall Davis and Toot and Stan Dunham.
Here is an article from the Huffington post which might be the one you’re thinking of:
http://www.huffingtonpost.com/2009/05/30/stanley-dunhams-wwii-expe_n_209385.html?view=screen
which ends with the statement:
‘But Ralph Dunham is reminded of his brother every time Obama’s face appears on TV or in the paper.
“You know,” Ralph says, “he looks exactly like Stanley. He looks exactly like my brother, only he’s dark.”’