I've written a custom plugin for CKEditor--successful on all fronts, save one currently: I can't, for the life of me, figure out how to customize the image on the button in the toolbar of the editor. As I'm a new user, you'll have to click through to see attached image; the highlighted square in the top left should have a pretty picture (like most of the other toolbar items).

![Screenshot](https://i.stack.imgur.com/CYg4y.jpg)