Download the Apple Ranbow image and save to your pictures folder. With the GeekTool window open, drag a new image geeklet to your desktop. Browse to your image file’s location by clicking Set local path… Select Keep on Top, so that your image will hover over the menu bar. Set the image’s position to the upper left of the geeklet; this won’t move your image to the top of the display, but it will make it easier to set your geeklet image’s final position later. Designate a nice long refresh rate so your Mac isn’t constantly using resources to reload the image, I used 99999.
On the geeklet image property window, change the size to something small, on my mac I used 65x20 and got the exact same size as the black apple logo. Move to the top left of your screen, as high up as possible, usually directly under the menu bar, then use the "arrow buttons" on the keyboard to move the image up and over to cover the current apple logo.