It's taken far too long for me to realise this. When you take a screen grab using one of the methods Apple describes (see this blog, below and this useful article , the image is saved to the clipboard.
I have been opening Preview and doing cmnd-n to see the screen shot in a new Preview window. Then it can be saved as an image in whatever format you please. But if you want to paste the screen shot straight into a word processing document or a presentation then you can simply do cmd-v or Paste from the top menu, and there will be your image.