Comment on Screenshots Preference Pane

In Mac OS X 10.4 and later, the default screenshot format can be changed, by opening Terminal (located at /Applications/Utilities/Terminal) and typing in:

defaults write com.apple.screencapture type image_format
killall SystemUIServer

Where image_format is one of jpg, tiff, pdf, png, bmp or pict (among others). If you omit the second line, you will need to log out and in again for the change to take effect.

http://guides.macrumors.com/Taking_Screenshots_in_Mac_OS_X