You could just have asked me directly.
- Have the desired screen "on top"
- Press the "Print Screen" button
- Open a graphics program like Paint (I use Photoshop)
- Create a new, blank document
- Go to the Edit menu and select Paste (or hit Ctrl+V)
- Edit the image
- Save it as a JPEG
- Voila!!
These instructions are for Windows operating systems. I use crop and resize to get the image to no more than 800x1000 pixels as per Fauna's attachment policy.
When you are ready to use it in a post, selct the "Manage Attachments" button under "Additional Options", "Bowse" to select the file, "Upload" and it will be there.