Graphics that flash at a rate of sixty times per minute or greater can actually trigger seizures, especially if they are bright red or orange. I was creating a Web site for my mother and used an international flags bar. This bar flashed several different flags, many with red, at about seventy times per minute. When I showed my mother the site she nearly passed out. Another point about graphics is size.
Small graphics are hard to see.
Large graphics take up too much space. Somewhere there is a happy medium. This depends largely on the size of the monitor and the screen resolution being used by the viewer. Try viewing your pages at several different resolutions and, if possible, on various size monitors. Also, view them with different browsers in the same fashion.