Alright well there is another way you might get this working. Look into your .fluxbox directory and check for a file called apps. If it's not there just create a new one.
In that file add the following line
[startup] {bsetbg -f /path to your pic}
Save the file, log out and then log back in and that should do it.
I believe you'll need the lastest devel. ver of Flux for this to work which should be 0.9.8 or close to that.
My Desktop