I added the following command to my ~/.fluxbox/init to have fbsetbg take care of the background setting:
session.screen0.rootCommand: fbsetbg -f ~/.fluxbox/backgrounds/SiamesicRobots1600.jpg
It does set the background, but I'm getting the following popup message every time I start Fluxbox:
fbsetbg: Something went wrong while setting the wallpaper.
Run 'xv -max -smooth -root -quit /home/me/.fluxbox/backgrounds/SiamesicRobots1600.jpg' from an xterm to find out what
So I pasted that command in the terminal and got the following:
me@laptop0:~$ xv -max -smooth -root -quit /home/me/.fluxbox/backgrounds/SiamesicRobots1600.jpg
X Error: BadWindow (invalid Window parameter)
Major Opcode: 4
How do I fix this?