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: