how to take a screen shot
How do I take a screen shot on my macbook pro?
Command ⌘ + Shift 3 or 4
3 for full screen, 4 for cross air selection
More here, including windows etc.
Import into Preview and save as a different format like jpg.
Also if you want to pernamently change the screen shot format, you can use Tinkertool or OnyX from Macupdate.com
(10.8 users hold control key or right click to "open" non-Apple tied software from the web and elsewhere)
