Specify what file format you want to use for your screenshots (shift⇧ + cmd⌘ + 3) under Mac OS X. Th default format is .png, there are a lot of additional file formats available: png, pdf, tiff, pict, jpg, bmp, gif, psd, sgi and tga.
defaults write com.apple.screencapture type *format*
Replace *format* with your selected format that you wish to change the default setting to (use: png, pdf, tiff, pict, jpg, bmp, gif, psd, sgi or tga)
To make the command take effect, you need to restart the System UI server:
killall SystemUIServer
Revert to the default setting (png):
defaults write com.apple.screencapture type png
Screencast:
How wonderful. This is really great!