Actually, it's neither one. It's a whereisit?
The "who" was Irene Adler. The "how" was by possessing letters from a member of the Royal Family to a Shady Lady. And the "where" was a safe in her sitting room. Sherlock faked a fire in her house and tricked Irene into revealing their location.
And just like in the BBC series Sherlock, the "why" she did it was not really to extort blackmail money -- it was the possibllity of future influence over the high-ranking official who had written racy laters to some hot chck. Remember, this was the nineteen hundreds. The world was prudish.
My, my, how times have changed. An international scandal -- over a few letters.
Wait . . . by God, maybe times haven't changed! I seem to remember more than one elected official being embarrassed by dirty emails he sent to various women because he didn't seem to realize that things which have been put on the World Wide Web tend to accessible to . . . well . . . the whole wide world . . . on the web!
Like, duh!
Damn! And we actually ELECT these idiots?
The more I read that story, the more questions I have about it.
According to the Bohemian prince, Ms Adler plans to ruin him by showing the photo of them to his royal fiancee's family, and she may have actually planned to do this as threatened. But by the time Holmes is actually on the case, she is engaged to Mr Norton, a much better person, and no longer has any reason to harm Mr Ormstein. In fact, it seems she is now more interested in making him leave her alone, which holding onto the photo is not helping with. Instead, he has been having her home burgled, her luggage diverted, and heself waylaid. He's beginning to look more like an abusive ex to me, and I think to Holmes as well by the end of the story. I love his remark about how "she seems indeed to be on a very diferent level" from him. This is one of several stories where Holmes is engaged by one client, but ends up helping someone else (often a woman) more.