Replacing WordPress Icons (menu,icons32, etc)?

The CSS is in wp-admin/css/colors-classic.css and wp-admin/css/colors-fresh.css and the min versions of those, including the icons32-2x.png ones. I can see them when I grep the directory. For example, wp-admin/css/colors-classic.css:2162.

The images themselves are in wp-admin/images, as you can see from the style rules, but you should not be hacking/replacing those. They will be overwritten when you update WordPress.

I don’t see the need for Javascript at all. You can register stylesheets for the backend. That should be all you need.