When I hit "PrtScn" the screen blinks hinting a screenshot is being taken, but then I can't paste it into Gimp. It says there is nothing to paste.
Tails saves print screen images in /home/amnesia (your home directory)
To find the images go to: Applications > Accessories > Files and look for the screenshot file ending .png
Or in gimp you open it through File > Open and it should be there.
https://tails.boum.org/blueprint/doc/screenshots/ Also, this link backs up what I said and explains a whole lot more.