Browsers load images after an HTTP request is sent for them either by <img> tag or through a javascript call. If you have a javascript function that changes an image on mouseover, you have to wait while the image is retrieved from the server. This would be disturbing when users have a slow internet connection or when the images being retrieved are very large. Although most of the browsers store images in the local cache so that subsequent calls to them are responded immediately, there is still a delay in the first time the images.. Read more..