Replace default icons sprite

I am attempting to make a plugin that uses a modified stylesheet for the admin. That works so far, but I want to override the menu.png file that is used for the icons with one of my own. All of the other CSS properties work from my stylesheet. Replacing the file in the core is the only way that I have been able to get it to change the icon sprite.

When I add the background: url(‘theurltothesprite’) ; to any of the icon properties it gets ignored and the sprite that is cached is still used which is – ‘../images/menu.png?ver=20120201’.

Does anyone know of a function or anything that I could use to force it to use my sprite instead for all menu icons? It is a modification of the original sprite.