The screenshots in a MAC OS X are saved in the PNG format, which is the format of choice for publishing and sharing images on the internet. On the other hand, JPG, TIFF, PDF or GIF formats can also be used.

Change the Default Image Format for Screenshots in MAC OS X

The Default Image format cannot be changed by simply following the conventional method i.e. from the GUI (Apple menu> System Preferences). In order to alter the settings, it is needed to run a particular command in the Terminal.

 Firstly, use the syntax for the command i.e. defaults write com.apple.screencapture type (format)     

Mention your desired file extension instead of (format)

Save Screenshots as JPG

JPG is overall a better option offering better saving photos and realistic images options than the PNG format. The JPG format usually creates images that are smaller in file size.

Open the Application folder> Utilities> Terminal. Run the defaults write com.apple.screencapture type jpg command followed by killall SystemUIServer

Save Screenshots as GIF

GIF has the advantage of being the smallest in size as compared to other formats. Moreover, GIF files offer to deliver animation and transparency.

Go to the Application folder> Utilities> Terminal.  Run the default write com.apple.screencapture type gif command followed by killall SystemUIServer

.The killall SystemUIServer command is used to restart the UI of MAC OS X, which will then imply your new settings.