Possibly, the difference is not that much as it automatically scales anyway. However, 480 x 234 is the true native resolution....
When the Z* hard drive is hooked up to your PC, you can right click on any of the screen images to be found on the 17.5 gig partition under "BG" and select "Open with". From the drop down list chose your photo editing software (Photo shop). Here you will see that the images are indeed all 500X240. All Z1, Z2, and Z3 images have always been 500x240.
No need to confuse anyone with some theoretical values.
Using the above method you can change the CPYCUSTOM.BMP to be found under BG/COPYRIGHT, by simply painting with the black background color over anything that you don't want to show up.
You can also add a logo, your License plate or your name so that it is always visible over any image that is displayed as your opening screen.
I just left the Tele Atlas info, moved it close to the bottom and added a small VW logo.
