I've been trying to figure out how to do this withought using the fluxbox-generate_menu script, because the script adds a bunch of unnecessary clutter and I only like to have certian programs in the menu. I searched the fluxbox documentation and man pages, but I finally found what I needed in the fluxbox 0.9.10 changelog.

* [wallpapers] (directory) {rootcommand}
creates a submenu in the menu to click on the wallpaper one wants. default rootcommand is fbsetbg. This works great!

Now I just need to find out how to get it to sort the wallpaper so it shows a different menu for each x number of wallpapers like the fluxbox-generate_menu script. For example:

wallpaper1 > 1.jpg
2.jpg
3.jpg
etc...
wallpaper2 > 4.jpg
5.jpg
6.jpg
etc...

I would like to do this without doing it the obvious way, which is to seperate my wallpapers into different directories.