Delete File “com.apple.finder.plist”
Users => Computer Name / User name => Library => Preferences => com.apple.finder.plist

To take a picture of the whole screen, press Command-Shift-3.
To take a picture of part of the screen, press Command-Shift-4 and drag to select the area you want.
The picture is saved as a TIFF file on your Desktop. Note: Mac OS X 10.2 and later saves pictures as PDF files.
To copy the screen capture to the Clipboard instead of saving it as a file, press Control-Command-Shift-3 to copy the entire screen to the Clipboard, or Control-Command-Shift-4 to copy the selected range to the Clipboard.

Change Screen Capture Format:
Run Terminal Command ->
defaults write com.apple.screencapture type jpg
Logout Or Restart

Support Format:
JPG/GIF/PNG/PDF