They crash some browsers and annoy people. They cut down on the loading speed of your page. Set yourself apart from the crowd by using original thought and original artwork.
Websites are by their nature constantly changing.
A stagnant website is a bad thing because visitors will never return. This makes the all too common black and yellow signs redundant. If your pages look great but take too long to load no-one will stay around long enough to read the content. Keep the text on your pages short and to the point, then add a few fast loading pictures that complement your message. That way the visitor only waits for that 100K image to load if he specifically wants to see it.