Rollovers are just images that are swapped with another image when you rollover them, such as buttons that light up when you are over them to indicate that they are buttons.
You can always make a rollover with the Insert>Interactive Images>Rollover Image object.
But, if you want to make them yourself, you can do your own Javascript in another way as well, through the Behavior Panel.
Making a Rollover
1. Select the image in your document that is the start image and name it in the Properties Inspector. Press Enter (Return) to complete.
2. In the behaviors panel, select a browser version 3 and above.
3. In the document window, select the img.
4. In the Behaviors Panel select the Swap Images action.
4. In the dialog box that follows, select the name of the image you want to swap. Enter the source for the new image. Select the Preload Image check box and the Restore Image check box if you want the image to go back to the first image on MouseOut, and click o.k.
5. In the Behaviors Panel select an event from the pull down (ex. onMouseOver or onLoad or onClick).