Because I was curious, I put my SA1 Dreamcast disc into my 13 year-old PC. And when I browsed through the "EXTRA" folder, I was greeted with desktop backgrounds featuring various Sonic Adventure characters (including the cute little Chao!)

The images will be at the very bottom of this post. But first, here's how to get it on a Windows computer:

1. Right-click on the image you want to use as your desktop background.
2. Left-click on "Save As".
3. Select the destination for your picture and the name of it. Remembering the destination is important when importing your image as your desktop background.
4. Once saved to your computer, click on the Start button (on Vista/7, it's the Windows logo inside a blue circle), then click Control Panel.
5. In the Control Panel, click Display (switch to Classic View if you have to) (In Windows 7, you search "desktop background" and click Change desktop background).
6. In the Display Properties window, click on the Desktop tab.
7. Click "Browse.", then locate the image you downloaded.
8. Adjust the image so it suits you (you can skip this step if it's perfect already).
9. Click "Apply". To close the window, click "OK".

Here's how to get it on a Mac OS:

1. If you currently use the mouse that came with your Apple computer, disconnect it now and replace it with a right-click USB mouse. Plug it into an available USB port in the back of your Mac.
2. Right-click on the image you want to use as your desktop background.
3. Left-click on "Use Image As Desktop Picture". You should now see your selected image on your desktop.
4 (optional). If you ever want to change the way your desktop image looks, click System Preferences at the dock, and select "Desktop & Screen Saver" in the System Preferences window.

Here are the promised images (they had to be uploaded in JPEG format because you can't upload bitmap images):