I hesitate to do anything fancy to my computer, reason being that a lot of sites that offer you free screensavers, smileys and other 'accessories' don't tell you that you'll be inviting a host of viruses in, too. I just had a virus scan last night, thanks to my trusty Norton Internet Security, and found at least 200 bugs which came as a result of some freebie thing I downloaded. Fortunately, they were detected before they could do any harm.
What is wallpaper, anyway? Is it downloadable? If it is, I'd use caution.