In case your customer enters a URL which is in the scope of your domain base path, but not referenced by any content you published, they would be redirected to a 404 page.
In most cases, site would provide a default 404 page. If you'd like to provide your user a better experience, you can create a file 404.md in the root folder of your docset, then OPS will bring user to the 404 page you specified that might contain instructions on what to do next.
Referring to an image by relative path is not supported in customized 404 pages. As workarounds, you can either use absolute URL of the image or embed the base64-encoded image.