I have exactly the same issue. When I open a photo or when I exit from the photo view I see those flashes. It flashes only once, but it happens every time I open a photo or exit from the photo view.When ever I click an image in Facebook to see a larger view, my screen is flashing like crazy.
I only have safari 6 installed so unable to confirm if its a Safari 6 issue.
Anyone else suffering from this ?