That's why you need to be careful when choosing your Avatars. Note that static images don't cause the problem, it is only the animated ones. There are 3 main triggers that cause this problem. Firstly, the speed/frequency of the animation can cause issues with people who are susceptible to this condition (which is a form of epilepsy), although the trigger speed can vary between individuals. Secondly, the actual image itself and what part of it is being animated (e.g. a striped ball which is spinning). Thirdly, the colours used in the animation, with alternating colours of high contrast being the most problematic. The more of these features present in an animated image, the more likely it is for a sufferer of Photosensitive Seizure Disorders to be afflicted.