If you haven't heard/seen it already then check out this story - http://www.bbc.co.uk/news/mobile/entertainment-arts-13785393
Short version: there's a painting which 99+% of the artworld reckons is a Monet. 2 brothers reckon it isn't. If they agreed it was, it would be worth (literally) millions - as it is the owner paid £40,000 and most people reckon that's too much.
So....almost nobody on earth can tell that it's not a Monet. But it's worth a small fraction of what a Monet would be.
Yes, it matters that this picture was taken by Gursky. In the same way it matters whether it was Rothko who sloshed the red paint around.
There was a program ages ago about how "experts" and in particular the Saatchi gallery set the price for art. They decide Damien Hirst's shartk is worth £50,000 - and all of a sudden it is. 3 days ago, Gursky had only taken the 3rd most expensive photograph ever. Now he's number 1. And I bet the price of his next print of 99 cent II Diptychon has just gone up too.
Oh and I have a shed. 2 actually. And one of them's huge.