The downsampling also isn't that great.
Original image: https://raw.githubusercontent.com/haasn/cms/master/rings_lg_...
Downsampled with imgix: http://chen.imgix.net/rings_lg_orig.png?w=400
Downsampled with imagemagick: https://0x0.st/1-.png http://i.imgur.com/Nvl7tAm.png
Downsampled with imagemagick, gamma correct: https://0x0.st/1i.png http://i.imgur.com/Hrm4COb.png
Note how the luminance becomes square in the center (step back a bit if you can't see it), and also the edge pixels on the imgix version.