In Mac OS X, the default screenshot format is .png If you want to change the screenshot format, follow these steps.
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1. Open Terminal.
2. Type this command and press Enter key.
defaults write com.apple.screencapture type image_format_you_want
Replace “image_format” with the file type you want. Ex: jpg, tiff, bmp, pict…
defaults write com.apple.screencapture type jpg
3. Log out and log in again (from the Apple menu) for the change to take effect.