I'd like to compress my screenshots as much as possible before I send them via email. The best compression ratio I only got with some work on each screenshot:

• first I took the screenshot with shutter: 1920x1080px, PNG compression 9 -> around 450Kb


• then I uploaded it to http://tinypng.org/ -> got the same picture with quantisation but only around 150Kb left

how could I achieve such a good compression within a screenshot tool like shutter?

The technique of http://tinypng.org seems fabulous:

similar colours in your image are combined. This technique is called “quantisation”. Because the number of colours is reduced, 24-bit PNG files can be converted to much smaller 8-bit indexed colour images. All unnecessary metadata is stripped too. The result: tiny 8-bit PNG files

But shutter has no "quantisation" option.

The Quality is not that important, I wouldn't mind to have it more lossy. Maybe with GIF it would be possible to compress smaller? Or with less colors?