defaults write com.apple.screencapture type -string "png"
Other options are allowed: BMP, GIF, JPG, PDF, TIFF.
An other nice move is to place your screenshots directly on your desktop:
defaults write com.apple.screencapture location -string "$HOME/Desktop"
And if you want to remove the shadow from the captured windows, just use the following stance:
defaults write com.apple.screencapture disable-shadow -bool true
Now, to take advantage of screen capture in your Finder, you can use the following key combinations:
- ⌘+⇧+3: Capture the whole screen.
- ⌘+⇧+4: Capture a selection of screen, end the selection via ␣ (the SPACE bar key).