How to find your screenshots on minecraft PC version 8.1 when you only have the demo available?

How do you find your screenshots on minecraft PC version 8.1 when you only have the demo available?

It should be in your minecraft directory in your program files C: drive

If you're taking screenshots through the game, then I can't help you. If you're taking screenshots using Windows (Win Key + PrtScn), then it saves them to a Screenshot folder in your Pictures library.