Do you have a file called lastwallpaper in your ~/.fluxbox directory and, if so, does the file have something in it? If not the -l is getting nothing so that's what you get on the screen. Know what I mean?
I'm on my laptop this time, which is a bit more tweaked. In my .fluxbox/init file "session.screen0.rootCommand:" is empty. It is setup to use a style and at the end of the config file for that style I have:
That was a major change in Fluxbox-1.0rc. It's mentioned in the release notes.
What I would probably do is:
1-pick a style
2-copy the file from /usr/X11R6/share/fluxbox/styles to ~/.fluxbox/styles (rename it if you want to)
3-change the init line session.styleFile to point to it
4-edit the copied file to include the background.pixmap line at the end.
You can also edit other lines in the style file to change colors and button effects and more. For example I copied the one called Twice to my directory and and added my background. Then (over time) changed the colors, button sizes, alpha to make it just the way I wanted.