To get the best quality screenshots - I'd do a screenshot from within playmode. If you need to arrange things a bit, do it like this:

1. Enter playmode and get to a certain desired configuration.
2. Pause the game.
3. Make any adjustments you like - repositioning in the scene view and adjusting inspector values.
4. Position the scene view camera for the shot.
5. Select the currently rendering camera in the hierarchy.
6. Match the rendering camera up with the scene view camera via "GameObject/Align with view" (shift + cmd/ctrl + F).
7. Take your screenshot.
8. Repeat from 4.

For when "High Quality" means "lots of pixels", you may also want to have a look at this forum posting: large screenshots for print

megafiers has a utility called 'megagrab': http://www.west-racing.com/mf/?page_id=401

Here's how I would do it:

1. Set up the scene how you want it for your screenshot.

2. Create a new camera, and position it where you want to take the screenshot from.

3. Open up a game window by going to Window > Game or pressing Ctrl + 2 (for Windows, on Mac it would probably be Cmd + 2).

4. Make sure the Gizmos button at the top-right of the Game window is toggled off, and take the screenshot from there (no need to hit Play).