You will need to carefully edit the onmouseover script of the Interactive Button,
since creating an Interactive Button (w/ the hover state already creates an event)
FP creates the onmouseover event in the image tag:
onmouseover="FP_swapImg(1,0,/*id*/'imgX',/*url*/'path/imagename.ext')"
and you would need to add a function - say "swapotherImg" as
onmouseover="FP_swapImg(1,0,/*id*/'imgX',/*url*/'path/imagename.ext');swapother(...)"
Where swapother is a function (with parameters tot he other image name) like you can find at:
http://www.developer.irt.org/script/813.htm
(you will also need an onmouseout event)
SP
If you edit the Interactive Button in the future you will need to redo the changes
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| FP2003, How can I make a small image show on an existing page when hovering over an existing interactive button?