The best way to do this is to adjust your monitor contrast to maximum to get the brightest white, then reduce it until you get separation between the white and light grey blocks on the right. The white box should be bright white (maximum contrast in most cases). Next, turn down the brightness to get the blackest black you can while still being able to distinguish the next brighter shade of grey on the left. The last block to the left should be black and barely separable from the dark grey to its immediate right. If they are easily separable then your monitor settings are to bright. With your contrast and brightness set this way you will not be missing any detail in the shadows or highlights of photographs.

There is currently no way to standardize colour using jpeg files over the Internet so the best you can do is set your monitor colour depth to display as many colours as possible. The bar above should show shades of gray. If it looks like it has a color, adjust your monitor until it does not.