As support for CSS improves, pseudo-selectors like :hover, :active and :focus will become more widely used. Already :hover is in use on many sites to provide rollover states to buttons, as on this site (the menu bar). The other pseudo selectors will, in time, give far more opportunities for the use of rollover images.
One potential problem with image rollovers, though, is that in order for an image to be displayed, it must be downloaded. Consequently, for rollovers to work smoothly and quickly, all the necessary images must be already available on the user's PC. Otherwise, the rollovers will behave badly, like in this example using large images.
So there's a clear need for a way to use CSS to preload images or find another way to avoid the problem. Which gives us two relatively simple solutions to our problem.
The first solution is to create a single background image for your element that actually contains both the rollover and non-rollover images, and then position is using the background-position CSS property. Instead of changing the image when the mouse moves over the element, you can simply change the background-position to reveal the previously hidden rollover image. There's a more detailed explanation of this technique over at WellStyled.com.
The other option available to you is to trick the browser into downloading the image before it is required for the rollover. This can be done by applying the image as a background to an element, and then hiding it using the background-position property. The image will then be downloaded but will not be displayed. Then, when the rollover is activated, it will operate smoothly and instantly.
First, you need to select an element that doesn't currently have a background image. If so select an element that does have a background image, you will either end up not preloading the image you are after, or you will prevent the element's normal background displaying. Neither is ideal.
Once you have picked an element to use for this purpose, you need to add the background image. The following CSS can be applied to the element and will place the background image outside the viewable area of the image:
background-image: url("rollover_image.png"); background-repeat: no-repeat; background-position: -1000px -1000px;
Your rollover image will then be loaded when the page itself is initially loaded, along with the other images. When a rollover is then activated, the image will already be available to the browser and the effect will be instant.