Simple via import
import screenshot.png
Or via scrot
apt-get install scrot
Basic usage
scrot # simple printscreen
scrot -d 5 # with 5 sec delay
scrot -s # select rectange or window
scrot screenshot.png # into specific file
scrot -h # help
Bind in i3wm
append i3wm config
bindsym $mod+F11 exec scrot
bindsym $mod+F12 exec scrot -s
bindsym Print exec scrot
bindsym $mod+Print exec scrot -s