First you need a digital picture.
A picture from your digital camera, or scanner, or you can download one from a web site.

To do web pictures, merely right click on the picture and click on "Set as Wallpaper".

To use a picture of your bus it needs to be in the JPG format, exactly 11.11 inches wide by 8.333 inches wide.(A windows requirement.) Most digital editing programs have the ability to resize pictures. Save the picture as a JPG with a name you will remember (MyBus.jpg or ??).

This works in WINDOWS ME
I haven't been able to try it in any other version so far.

After the picture is saved go to the folder it is saved in and copy it, (Right click and Control "C") then open Drive C:.
Open the "Windows" folder and find another folder called "Web". Open it and find another folder called "Wallpaper". Open it and paste (Control "V") your picture here.
Go to your desktop.

Right click on your desktop, choose "Properties". A window should open with the "Background" tab already selected. Scroll through the selections available until you find your "Bus.JPG" . Highlight it and click "Apply", at the bottom of the window. Click OK.
Close the "properties" box and sit back to admire your handiwork.