image format for Screenshot

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http://askubuntu.com – In 12.04, can the image format for Screenshot be jpeg? I change the file name to save to to be ended with ".jpg", but Gnome image viewer seems not be able to open it saying Error interpreting JPEG image file (Not a JPEG file: starts with 0x89 0x50) One example image is here http://dl.dropbox.com/u/13029929/c1p18.jpg Is it a true jpeg image file? Thanks! (HowTos)