When you take a screenshot in OS X, using the default button combination, you’ll notice that it’s saved in PNG (Portable Network Graphics) format.
You may prefer another format, such as JPEG, but it’s not obvious on how to change it.
From within a Terminal, you can change it pretty easily:
defaults write com.apple.screencapture type jpg
This would change the default format to JPEG, but you would need to log out and log back in for the change to take effect.
There are other formats too – Tiff, PDF, and the default PNG. You can change to any of them by changing “jpg” on the command above to whichever you require, by using one of the following:
defaults write com.apple.screencapture type tiff
defaults write com.apple.screencapture type pdf
defaults write com.apple.screencapture type png
Remember after changing the format, you will need to log out and log back in to OS X, for the changes to take effect.