For me, it overflows even on desktop. The issue is that your set size of 800px is to wide and there isn't any global CSS to shrink the image or hide the overflow. You are on the right track trying to apply a max width of 100% but the image syntax doesn't accept custom style attributes. What you can do is add class and then add CSS to the wiki's stylesheet to shrink the image where needed.

Image Link

[[File:Killing-Stalking-killing-stalking-40176415-1200-600.jpg|800px|center|class=shrink-x]]

Wiki CSS

.shrink-x img {
    max-width:100%;
    height: auto;
}

Thank you sm!!