This tools captures to current output of the Raspberry PI and writes it as JPEG to stdout.
$ ./screenshot > snapshot.jpg
By default jpeg writing uses a quality level of 75. This is probably a good compromise between quality and file size. To change the quality level just provide an parameter:
$ ./screenshot 90 > snapshot.jpg
You will need to install libjpeg before you build the program. On Raspbian
$ sudo apt-get install libjpeg8-dev
Then just type make in the screenshot directory you cloned from github.