Many displays are not calibrated well to do that test justice. Also if you have a high DPI display, then the browser could already scale that picture for you, screwing with the test. You should display those images with image pixels matching device pixels.

You can visually test your display calibration with [1], but you should ensure that the image is displayed 1:1, which might not be the case with your browser and hiDPI settings.

[1] http://www.lagom.nl/lcd-test/gamma_calibration.php#gamma-tes...