In the config file at

~/.config/i3/.i3status.conf

you will find two lines that will help you make screenshots:

# Screen Shots
bindsym Print exec scrot '%Y-%m-%d-%s_screenshot_$wx$h.jpg' -e 'mv $f ~/Pictures/'
bindsym Control+Print exec gnome-screenshot -i

The -i after gnome-screenshot makes it interactive as shown in the picture below.

Both of the them will be shown in the tutorial.

Picture of gnome-screenshot (interactive or -i)

Tip : you can change gnome-screenshot save the picture in JPG rather than PNG. That saves a lot of megabytes.

More info here.

gnome-screenshot