If you're planning on using jpegs instead of TGAs, then enter
jpeg_quality 100
and the command to take a screenshot is
jpeg
which you'll need to bind to a key so that you can take screenshots without the console in the way.
A comprehensive list of commands to use to change everything to the highest settings might be hard to come by, so I would suggest saving your configs somewhere and then just cranking everything up to full in the GUI menu. You're going to want the max AA and texture quality, and put HDR on.