You must have noticed that either you didn’t have a wallpaper or couldn’t change the one your theme set for you. now asking your self how do i change fluxbox wallpaper.

Use this command to set the wallpaper (full screen).
fbsetbg -f /path/to/image.png

fbsetbg -c /path/to/image.png

or right click the image you want and select set as wallpaper. but this might only work on certain distros