If all you want to do is have an image as your fb background then follow what the first guy said, replacing backgroundimage.ext with the image you want as wallpaper.
There's a hidden folder in your home directory called .fluxbox (hidden files start with a . ) and inside that is a file called init which contains a line
Code:
session.screen0.rootCommand:
to make that set the image, change it to:
Code:
session.screen0.rootCommand: bsetbg -f image.jpg
where image.jpg is your wallpaper. bsetbg is a script to put an image as wallpaper.
If you want a random image then follow the advice I posted earlier.