It's been a while since I completed it but I think you have to go in the middle of the flowers and look out the window as though you would when selecting a flower
Did some reading after that post and apparently it's the picture frame you have to select.
It's been a while, but isn't the picture frame the one that leads you to the credits level? You get that one just for finishing all the levels I think, and it's not the same as the one you get for collecting all the rare flowers.
Today, February 12, we’re all very proud to be celebrating the one year anniversary of Flower’s release on the PSN!
we’ve put together a beautiful and serene dynamic theme, complete with day and night cycles, for all the fans who want to bring the Flower experience to their XMB. This went live on the PlayStation Store yesterday for the low price of $2.99.
Coming soon, we’ll be releasing the original soundtrack for Flower composed by Vincent Diamante.