Tonight I was lying down with my eyes closed in a dark room trying to ignore my randomly barking dog. Quick clarification for anyone not having met my dog -- she's a keeshond. They're bred for companionship and watchfulness (involving a sharp bark).
Anyway, if she was quiet awhile and then barked, I saw a flash of light. Kind of a horizontal line in white tinged with red at the edges. Sort of red-white-hot like the picture. It happened more than once. So I started to take note. If I lay on one side, so one ear was smooshed in the pillow, the lightning effect came from the side of the exposed ear. If the dog kept barking, it was only the initial bark following a period of silence that would cause the effect. So the startling the nerves had something to do with it, though I was never startled enough to actually jump or flinch.
I've Googled this tonight and found some other anecdotal stuff of people mentioning they sometimes see a bright flash behind their closed eyes when they hear a loud noise (and a blog survey by a guy asking what people see when their eyes are closed -- colors, flashing, grey shapes, squiggles, etc), but nothing neurological or scientific. I guess I'm kind of wondering if this happens to everybody, or if there's something weird and different going on in my brain.