I can't paste the screenshots I capture on Windows

You're better off setting the default action to copy the image, like normal PrintScreen does, so you can immediately paste. It also looks like you're rushing and not ensuring you have focus in something you can paste in, like Paint.Net which I just tested for you. Open that or fbMessenger or Skype or MSPaint or anything that can paste an image, make sure it's focussed, the Ctrl+V. -- The real problem here is that they've recently added that intrusive popup and now it's impossible to take quick successive screenshots, because taking an immediate second one snaps the small popup. This mistake should be removed, since it cannot be removed in Notification settings.