Change Apple's default screen capture file format
Change the default image format of Apple's built in screen capture. By default the screen shot format is saved as a "png". This example shows how to change the default format to a "jpg".
1. Open Terminal.
2. Copy the "defaults write" line below between the ``
`defaults write com.apple.screencapture type jpg`
3. Paste it into the Terminal window, and press Return.
4. Log out or restart your Mac to make it pick up the new setting.