Fluxbox, like Blackbox, just has two wrapper utilities for this - fbsetroot and fbsetbg. Let's see how they work.
fbsetroot is about the equivalent of xsetroot, which can set backgrounds as long as they're simple, like a solid color. fbsetroot can also set gradients.
fbsetbg is a wrapper that tries to find a suitable background-setting app and then tries to set the wallpaper using that app. You don't have to configure fbsetbg. It just uses the first app it can find.
Fluxbox allows to ignore the background specified in a style file by specifying it in your ~/.fluxbox/init file:
session.screen0.rootCommand: fbsetbg -f ~/backgrounds/zimdib_dark.png
To avoid having to change the ~/.fluxbox/init file, fbsetbg will store the used wallpaper in ~/.fluxbox/lastwallpaper and a reload it when called with option -l. The rootCommand should then be.
session.screen0.rootCommand: fbsetbg -l