How to change the format of screenshots saved to your Desktop on Mac OS X
Mac OS X has several options for taking screenshots with shortcut keys or taking screenshots from the command line. By default these screenshots are copied to the clipboard or saved to your Desktop as PNG files. You can set these saved screenshots to one of several different filetypes using one of the commands below.
To change the default screenshot filetype, open a new Terminal window and enter a command like this (replacing "{filetype}" with your preferred filetype):
defaults write com.apple.screencapture type {filetype}
... then log out and back in to activate the changes.
Permitted filetypes are "jpg", "png", "bmp", "gif", and "tiff".