Image upload errors/glitch?

Hello,

I found problem in 3.8.2 and 3.8.1 WP version (maybe there was the same problem in latest versions too).

I use test wordpress instalation with twentyfourteen theme (problem does not depend on used theme). When I upload images with absolutely clear background (e. g. tshirt products) WP make something with their quality.

I made example:
1) Uloaded absolutely clear image with 1038 x 568 px.
2) WP made preveiws/thumbnails with sizes:
– 1024 x 568 px
– 672 x 372 px
– 300 x 166 px
– 150 x 150 px
3) Look at results here (high resolution photo). On the left side is originaly generated previews/thumbnails – errors have light grey color, almost invisible. On the right side is 100% contrasted originaly generated previews/thumbnails for better illustration.

I have some projects with Woocommerce plugin and its generated previews/thumbnails are absolutely ok. But WP’s not.

Is there somebody who knows why it happens?

Note: I tried to usu plugin for changing default compression value, but it doesn’t solve this problem. Images doesn’t look like compressed but glitched. Actually images with grey places couldn’t have less data than absolutely white images. It doesn’t make any sense :).

Thank you.