Mac: Change file format of screenshot

Open Terminal.app (/Applications/Utilities) and type the following command:

defaults write com.apple.screencapture type jpg

You can replace "jpg" with your desired file format (example: tiff, pdf, png, etc.). If you wish to change it back to defaults, Mac OS X originally grabs screens in png format. To activate the changes, just logout of your account and then log back in.