How to set wallpaper in wmii? I tried feh and xloadimage without any success. Is there any working example to set the wallapaer?
It's available in the Arch Linux [community] repository (which is what you're using based on the tags).
Just add the command to your wmiirc or xinitrc (of course you can also just try using it from a terminal first).
I always used "fbsetbg". It's a wrapper, so you may need to install some image manipulation software in order to get it working. (This wrapper is part of Fluxbox, so you should install Fluxbox too.)
Or you can use "feh". - feh is a lightweight and powerful image viewer that can also be used to manage the desktop wallpaper for standalone window managers lacking such features.
Few other apps - Feh and Nitrogen