Image compression issue

This one has compression
http://www.artrift.com/photo/12064/my-av4t4r/userid_2209/
http://www.pixiv.net/member_illust.php?mode=manga_big&illust_id=30223374&page=1

This one has no compression
http://www.pixiv.net/member_illust.php?mode=medium&illust_id=52880107
http://www.artrift.com/photo/18307/rinn/userid_2209/

This one also has compression
http://www.artrift.com/photo/18310/dimension/userid_2209/
http://www.pixiv.net/member_illust.php?mode=medium&illust_id=47185006