First. Install gnome-screenshot (Or any other screenshot software)
sudo apt-get install gnome-screenshot
yum install gnome-screenshot
Or you might already have it installed.
1. cd .fluxbox
2. nano (Or whatever text editor you want.) keys
3. add to the bottom: None Print :ExecCommand gnome-screenshot
You can change print to whatever key you want. (Print = print scr on your keyboard.)