I have been using Ubuntu 10.04 on my HP notebook for more than two years, and I have just installed Ubuntu 12.04 on Mac Pro.
The problem of taking screenshots on a Mac is that there is no PrintScreen Key. So how can I take screenshots?
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Open the dash and type "screenshot" you'll be prompted with an app, with a photo camera icon, that will let you take different types of screenshots. You can grab the whole desktop or grab the current window or Select an area to grab.
While that app is open right click on it's icon in the launcher and select "lock to Launcher" to keep it there.