If you would like to create your own button icon you can. The image must be a .gif image (if you are using Adobe Photoshop to create the images, then use "ctrl shift alt S" to create the image), and it must be equal to or smaller than 46 pixels wide and 36 pixels tall to fit on the button properly.
The maximum dimensions an image that fits on a GUI Button are shown below. Note that the image will be centered on the button. Therefore, images that are smaller than the maximum dimensions will work as well.