The rollover navigation slices on the left not only instruct the image being rolled over to swap, they also instruct another image to swap (the black interior slice of the shape above).
To create a Swap Image Behavior in Dreamweaver that
swaps an image other than the rollover image:
Select the graphic that is to be moused over to initiate an image swap.
Click on the Behaviors palette tab to select a behavior. Click the plus
sign button (add a behavior) and choose Swap Image. When the Swap Image
window opens, the name of the graphic you selected will be highlighted
in the scrolling area at the top of the Swap Image window. If you didn't
assign a special name to the placed graphic (in the Properties panel,
next to the little thumbnail image of the selected graphic), Dreamweaver
assigns it the generic name of "untitled" in the scrolling area
of the Swap Image window. In this scrolling area is where you select which
image you want to be replaced when the target image is moused over. So,
if you assign a name (using the Properties palette) to the placed graphic
that you want to replace, it will be easy for you to find it and select
it in the scrolling area of the Swap Image window. Then you can click
the “Browse” button to select the replacement image, or type
the file path and name into the "Set Source to:" field.