Well, you should have a recordset that populates the content on the page in question? On that page are already the initial images that you would be using as the 'off state' - you just need to modify that area to include the on state.
So for example, and using the example from the page I linked to above:
<a href="dog.html"><img src="images/<?php echo $row_Recordset['OffState']; ?>" onMouseOver="this.src='images/<?php echo $row_Recordset['OnState']; ?>'" onMouseOut="this.src='images/<?php echo $row_Recordset['OffState']; ?>'" border="0" height="100" width="100" /></a>
Obviously the above assumes that the images are located in the images/ folder - you would need to edit this to where your images are actually stored. 'OnState' would need to be renamed to the column name that is being used for the hover over image and 'OffState' needs to be renamed to the column name that is being used for the main image.
'Recordset' would also need to be renamed to whatever the recordset is that is being used to display the relevant product information ( you will need to make sure that the recordset itself is accessing all the columns required for the images).
Hope that this helps?