Here is some Tips For Wallaper designing.

Colors: use such cool colors which any can see them for longer....it means wallpapers should have propers colors which gives you cool look and anybody keep that Desktop for longtime....

Keep your themes: Keep your sub in focus and highlight them .... and your wallpaper should tell story as you mentioned in tittle....dont try to put any unnessesary things in your wallpaper...

Size: size is matter when you are posting your wallpaper....Genrally its 4:3 aspect ratio means (1600px x 1200px, 1280px x 960px) Try to keep your size 1600px x 1200px so everybody can resize them as their needs....
if you are designing wallpaper for HDTV screen then you should have your wallpaper's  aspect rastio 1.85:1 its for wide screen TV
if you are designing wallpaper for Dual Display then size should be 3200px x 1200px