if it works in all browsers except Internet Explorer then the culprit is probably Internet Explorer. I really don't like Internet Explorer and am really happy I never have to use it and feel sorry for all people using it. Anyway keep in mind that it may not work in some versions of IE but may work in other versions of IE.
If the example on my website http://iwebfaq.org/site/iWeb_Rollover_images.html
works in IE but yours doesn't then there may be an error in the code you're using. If the example on my website doesn't work either then you could try to add the code to your site using Alternative HTML http://iwebfaq.org/site/iWeb_Alternative_HTML.html
instead of HTML Snippets. If you already used Alternative HTML to add it and it doesn't work then you may want to try using another picture format (.jpg instead of .png or .gif or whatever). If still not working I don't know, sorry.