u mean its impossible to do something like that
It's impossible using Javascript. Remember, the entire world isn't running Microsoft windows. There are many people using Macintosh or Linux for example. How would you propose to set their backgrounds assuming it was even possible to do so. This is why you can't do it using Javascript.
It's not beyond the capability of what an Active-X control can do. But now you're talking about something that's much more complicated to do and goes
WAY beyond simple javascript coding.