I've a little problem : I use a transparent image to make round corner on main menu item. I specify a onbgcolor and a offbgcolor. The problem is that the offbgcolor never comes back after selecting the main item.
Yes, Maz is correct you need an overbgimage. But that still won't solve the problem, I tried it. The solution I found was to make the bgimage and overbgimage non transparent. That would mean you need two bgimages one 118px and one 73px, and three overbgimages, two 118px and one 73px. The third one you'd make green for the item you want to be green in mouseover.
One thing to remember, bgimages repeat. You have itemwidth and height set and if text size is increased in Netscape it won't make the items wider but it will make them higher and that forces the background image to repeat. You can avoid that by making the image about 100px high. The file size remains the same 1px. Hope that helps. If you need the images, I have them since I made them to test things.