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Read > http://support.apple.com/kb/HT3207 That option is only available for some old MacBooks Pro and Snow Leopard, so you can't do it in your MacBook Air.
In general, the MacBook Air isn't a computer to make photo editing. Its graphic is very poor and applications like Photoshop doesn't support it officially because it only comes with a Intel integrated graphic card. If you want better performance, you will probably need the MacBook Pro