Press the 'Print Screen' (grabs whole screen) button or Alt + Print Screen (grabs active window). This copies the screen shot to the PC clipboard. You need to paste the contents into an App such as Paint, or Photoshop then 'save as' jpeg (or if you are running the Picasa application it will automatically save a JPEG file in the Picasa / screen capture directory).
Then just treat as a photograph - upload to your image host or TP gallery and link in a post as usual using the [ img ] tags.