Image 
  Rollovers :
  
  Image Rollovers are when you roll over an image and that image is swapped with 
  another image. This works well with Button highlighting on Rollover. It is actually 
  produced with Javascript, but Dreamweaver does it for you. 
Three things happen in an 
  imagerollover:
  1. You need to Preload the images so they will be there when you rollover them.
2. When the user mouses over an image another image needs to be displayed.
3. When the user mouses off the image, the original image reappears.
Setting it up:
1. Make two images. One 
  for the Up state and one for the Over state. 
  

2. Click on the Insert Rollover 
  image in the Insert Panel or choose Insert/Image Objects/Rollover Image
  

or

3. Select the source ofthe original image and then the rollover image.
4. Enter an image Name.
5. Put in a URL if the image is linking somewhere when pressed.

6. Click ok.