Being a graphic artist is different than being a web artist or web designer.

What Type of Web Designer are You?

Anyway, here's my evaluation.
1. USD15-25
2. USD50 max (if you're lucky, someone might get it for USD100)
3. USD10-15 per wallpaper (Request more so bring down the cost)

The reason I'm using USD instead of RM is because I think Malaysians won't pay for a mere wallpaper. If you want to sell, aim at people willing to pay US Dollars.

Small cash but as your rep builds up, they might ask you to design something different.

Anyway, I guess it depends on rep but seriously if someone doesn't know how to use it effectively on their website then they won't purchase it. I wonder how much does Fotosearch or the contributors earn from Digital Art.

AFAIK, and feel, websites like fotosearch make more money from the photos compared to the digital art. But if you really want to know if you can make money from your hobby, why not sign up an account or auction the wallpaper off somewhere? Ebay? Fotosearch?

If you don't try, you'll never know.