Only a small part of the epileptic people is sensible to the flashing light. Some are affected by strong strip patterns instead. Most are totally not affected by this, and their seizures can be triggered by other means.
From https://en.wikipedia.org/wiki/Photosensitive_epilepsy
> Photosensitive epilepsy (PSE) is a form of epilepsy in which seizures are triggered by visual stimuli that form patterns in time or space, such as flashing lights, bold, regular patterns, or regular moving patterns.
From http://www.epilepsy.com/learn/triggers-seizures/photosensiti...
> For about 3 percent of people with epilepsy, exposure to flashing lights at certain intensities or to certain visual patterns can trigger seizures. This condition is known as photosensitive epilepsy.
[edit] Found this based on suggestion below. Obviously, don't click if you're seizure-prone: http://i206.photobucket.com/albums/bb217/powercrazy13/you-de...
I guess I'm not epileptic. It's pretty uncomfortable to look at for a long stretch though. Anyone know what happens if a non-epileptic stares at an image like this, say, for an hour?