Well, it works fine in firefox, but firefox is a bit more forgiving on syntax. IE is the only browser I see an issue come up. I would check all your code around the onclick event for the buttons and make sure there are no issues with the "down" image call. Also note your running in quirks mode. (DNN causes quirks mode by default) This means IE will rely on how it wants to display your code vs what standards are. This can be a bit of an issue when rendering javascript based images. You should probably force it into standards mode. Do a quick search on the forums here for how to do it. If you have a hard time just let me know and I will repost how to.

On a side note any reason for javascript based buttons / links? Why not flash buttons or even standard links with image properties set?

If you are using DNN 4.4+ I suggest using the .doctype.xml option for forcing standards mode. Basically all you do is name a file: skinname.doctype.xml and inside it put the doctype declaration. This way its independant to the skin. Making the changes to the default.aspx should work as well but I personally have had issues / times when I couldn't get it to work as expected with my 4.4+ installations.

Not saying it will fix your problem but could help I really think the issue is around the syntax. Mainly cause it renders fine in firefox. I don't know enough javascript though to be of much help. On that deparment though.