Disclaimer : This post is solely for educational purposes. Any outcome of misapplication is thus not my responsibility.
The windows operating system uses a bitmap image for wallpapers. An image of another format chosen as a wallpaper is first converted to a bitmap image and copied to a specific location and named wallpaper1.bmp.
For this tweak, we'll need a bitmap image (with a *.bmp extension). MSpaint and Irfanview are tools that can be used to create (or convert) such images.
Copy the bolded text below into notepad and save it with a .bat extension.
Copy /y wallpaper1.bmp "%userprofile%\local\settings\application data\microsoft"
Name your new wallpaper as wallpaper1.bmp and keep it in the same folder as the bat file we just created. Execute the bat file.
Just remember not to use an annoying image if its your friends pc you're doing this on - it might bore him/her. lol
Learn how to automate this action on every start-up of windows