The background switch CSS method is much better than using JavaScript to preload images. Create a single image with both the off state stacked on top of the on state. Then use something like this in your CSS:

Code: Select all

#imagetabred a {
    background: #f00 url("tab.gif") no-repeat 0 0;
}
#imagetabred a:hover {
    background-position: 0 100%;
}

That way you don't need to preload anything at all.