From the Tails UI, select Applications > Utilities > Screenshot
This will open up the Screenshot utility. Form here you can:
Take a screenshot of the entire desktop. Take a screenshot of the current window only. Select a specific area to take a screenshot of.
You can also change settings, such as:
Time-delay your screenshot. Include your pointer. Include the window border Apply effects.
Once you have taken a screenshot, you can name it and save it wherever you wish - including a USB if you wish to keep it off-system, perhaps with your USB Tails bootloader.
You can also use the Print Screen button on your keyboard to take a screenshot:
PrtScr (Prt Sc on some devices) will take a screenshot of your entire UX screen. Alt + PrtScr will take a screenshot of your current active window. Your screenshot will be saved to Home by default.