I think that a basket full of yarn is pleasing to the eye: it gives a feeling of comfort and warmth. I have baskets stuffed with yarn everywhere in my house, so why not on my computer?
Tonight, I discovered that Knit Picks has some desktop wallpaper, featuring their luscious yarns. Check it out here
Vogue Knitting also has knitting-themed wallpaper here
Hello Yarn has some gorgeous photos available here
So what is stopping you from giving your computer a warm, cozy feeling?