Topic: Make a photo that you think would work well as computer wallpaper, and try it out for a week!
Here my position will be very conservative. A wallpaper is to look subtle. It's just a background, and its primary function is to make sure your eyes' focus stays on other things. Almost equally important function is to give your eyes and brain something soothing to see. Soft curves, delicate colors in greens and blues.
Technicalities: used the shallowest possible DOF (50mm lens at 1.8). I needed to bump up the saturation and decrease the exposure by almost a full stop (a wallpaper can't be too bright--ergonomics, again). The weather was dull again, so I had to cope with that in postprocessing...
Practice photography at Daily Shoot.