To make a rollover-- an image that changes when the mouse is moved
over it-- you need two images: one the image that appears when the
mouse is not on it, and one the image that appears when the mouse
IS on it.
First create and save those images. Be sure to make them the same
size or the rolloveimage will be distorted.
Click at the point in your document where you want to insert the
mouseover. From the Insert menu, choose Images, then Rollover Image.
In the dialog box that comes up, enter:

- Image Name: a unique name for the image combination-- this
is used to make the image swap
- Original Image: the filename of the image that should show
when the mouse is NOT on it.
- Rollover Image: the filename of the image that should show
when the mouse is over the image.
- When Clicked, Go to URL: the filename or URL to link the
graphic to.
Click Ok to finish.
(taken in part from Lehigh
University's Intermediate Dreamweaver site)