My OS is Linux - I was using Kubuntu 17.10.1, a week ago I installed KDE neon (User Edition). This problem happened when I upgraded to LibreOffice v6 on Kubuntu and still exists on Neon. There are thin white horizontal lines on every image I put in Writer documents, but only when the image Anchor is set to “As Character”. Very rarely some of the lines disappear, but 99% of the time they exist. With LibreOffice v5 everything was fine.