You need to edit the brushes files, specifically those that handle icons. They way they work is as follows (buttons in this example) - there are several large files containing textures for all the icons and buttons in all their states (pressed, disabled etc.); the brushes files contain references to those files as well as coordinates (pixelbox) telling the game which part of the image to draw for the button. So if you add new buttons to the game, you need to both add the art in the appropriate file(s) (you can even make a completely new file if the old one has no more room) and then add entries in the appropriate .brushes files so that the game knows what to look for and where. And that's it! Hope it helps!