If you want to make your shots looks really good, change your renderscale (/renderscale #), and then resize the image to fit your monitor after the shot has been taken. The Default is 1
/renderscale 2 will take a shot at double your screen's default dimensions, 3 will be 3 times. Keep in mind, these are rectangular areas, so renderscale 3 is actually 9 times the total pixels.
You should not run renderscales much higher than 2 or 3, you WILL crash the game.
Most screenshots we take will be on a computer running at 1680x1050, and we'll take them at renderscale 2, and then resize afterwards. This gives a cleaner look to things, and provides a type of antialiasing.
So type in /renderscale 2 to get the better picture.
To return it to normal again type in /renderscale 1 ?