How to Change Mac Screenshot File Format in OS X
You can change the screen shot file format type on any Mac with any version of OS X by using a defaults write command string entered into the Terminal application. You can change the screenshot from PNG to JPEG (JPG), TIFF, GIF, and PDF.
Find the appropriate syntax to use here:
http://osxdaily.com/2010/08/16/change-the-screenshot-capture-file-format/
For example to change the screenshot file format type form PNG to JPEG, use the following defaults string syntax:
defaults write com.apple.screencapture type jpg;killall SystemUIServer
Once you have made the change, you can test the file format type by taking a screen capture with Command+Shift+3 or issue the equivalent of a Print Screen on the Mac by hitting Command+Control+Shift+3 to copy the screenshot directly to a clipboard. Complete commands list for screenshots in OS X is here:
http://osxdaily.com/2010/05/13/print-screen-mac/
This applies to all versions of Mac OS X, from OS X El Capitan, Yosemite, Mavericks, Mountain Lion, Lion, Snow Leopard, Leopard, Tiger, etc, it has never changed!
http://osxdaily.com