Using plug-ins enables selections of functions to choose from. Setup for what functions to be used is specified at the end of a file ~/.golem/golemrc . Remove # off from each plug-in statement you want to use and save the changes you made, and restart by right click enables to use specified plug-in. For each function of plug-ins, please refer to rc files in each plug-in directory under ~/.golem/plugin/ .
To specify a wallpaper, make sure the following are not commented out in ~/.golem/golemrc . If commented out, remove # to activate.
Check the following in rc file under the directory for the theme. If it does not exist or is commented out, activate it by removing # at front.
forplug "setroot" "forplug/setroot/rc";
To set a wallpaper use wmsetbg as default. Check if there is a directory /forplug/setroot under the directory of the currently used theme. If there is not one, copy from other themes.
cp -r ~/.golem/themes/eBlueSteel/forplug/setroot ~/.golem/themes/hoge/forplug/setroot
Place your wallpaper file in the directory setroot mentioned above, and edit rc to change All-Good-People-4.jpg to be used as wallpaper, save the change, and restart. 
In case you want to change wmsetbg options, change statements in ~/.golem/plugins/setroot/rc. wmsetbg is a command included in WindowMaker. Other commands, Esetroot, xv are available. When you use them, change aforementioned rc file as required.
When finished setting, right click on your mouse and then select restart. If the correct wallpaper is displayed, then it's done. Please refer to the help files for details about wsetbg commands and setting wallpaper for Esetroot.
 All-Good-People-4.jpg is a wallpaper for eBlueSteel