In the “Type here to search” box of the taskbar type in the following…
%userprofile%
and then press Return. That will take you to where your user profile is stored. By default, on an installation of Windows 10 on the C: drive, that will equate to C:\Users\Username\ but it’s possible to have moved the profiles location, so the variable will always take you to the correct location. Inside that folder you should find a Diablo III folder, inside of which there will be a Screenshots folder. That’s where your screenshots will be held. Whilst you’re in-game, press Print Screen to take a screenshot, then look in the folder.