Ok, I can look kindly on a cry for aid... goggle for logonstudio... there you will find free software, a fantastic selection of artful and wild images, and all the help you need. With a bit of thinking you will be able to take a logon image without the logon window and use it for a background. Seamless and professional.
Logonstudio is not the only site, goggle "customise XP logon", learn to build your own.
Alternatively, for a desktop background alone, find a suitable image that somewhat matches your display's pixel resolution in image size and dimension ratio eg mine is 1280 x 1024 pixels but even 1280 x 960 would do... save the image as a bitmap .bmp. Next go control panel, display, desktop, browse to your image and select it, apply n out. There!
Use nothing that your mother would not approve... :)