Disadvantages to this kind of rollover
This method of doing rollovers is very simple, but you should be aware that there are some real problems with it.
Using this method will cause an error message in older browsers, such as Netscape 2.0 or earlier, Internet Explorer 3.0 or earlier, or the America Online 2.7 browser.
Because the second image is downloaded from the server at the time the user rolls over the first image, there can be a perceptible delay before the second image replaces the first one.
Rather than use this rollover method, we suggest that you use the following way to create rollovers, which solves both problems.
Common rollover mistakes
Another common error is putting the onMouseover or onMouseout event handlers in the IMG tag. They must always be in the <A> tag, which is the Anchor tag that creates a hyperlink.