When using CSS image rollovers, two, three, or more images must be loaded (and often be preloaded for best results). We've got one image for each state (normal, hover, active, visited etc). Putting all states into one image makes dynamic changes faster and requires no preload.
Let's have a simple example. The menu items are the a-elements with display:block. Proper padding and background image for a, a:hover and a:active make the rollover. To simplify the rollover, I used only one picture containing three states of a button — normal, :hover, and :active.
Fig. 1: Three states together in one image
Usually, in CSS rollovers, we use background images in this way:
#menu a {
...
background: url("button.gif") top left no-repeat;
}
#menu a:hover {
...
background-image: url("button-over.gif");
}
#menu a:active {
...
background-image: url("button-active.gif");
}
/* etc... */