Thanks guys. The CSS version (from Brogan's post, adding a class to the logo image and then defining a hover CSS value for it in extra.css) works in the sense the other image appears on hover over. BUT, it seems to be UNDER the logo. So I can only see changes in the image through the transparent parts of the original logo when I hover over. I want the image to be completely replaced on hover over, or to appear over the original logo, otherwise this doesn't work. So still a bit stumped.
I'm only trying to make an Easter Bunny "wink" on hover over, so this is not mission critical.