When you mouse over an image, it is replaced by the 'rollover' image.
Normally the two images share the same colour, but in this case
I swapped them around to make things more interesting.
The images for these rollovers can be found here.
There are 16 gif images, eight colours-two images per colour.
When making your own rollovers, the most important thing is to make sure that both images are the same size. Don't use the Dreamweaver resize, as this only changes the display size. not the actual size. Use a graphics program to make sure both images are the same size.
To insert a rollover image go to the Insert menu and choose
Insert => Interactive Images => Rollover image.
The popup dialog allows you to browse to the original and replacement images, and also to define a hyperlink.
You will need to preview the file in a browser to see the rollover work.