Images that have been added to a course description do not appear on Course categories screen before login the first time. Such images are stored in a location with the HTML editor that is not accessible unless you have already logged in. After login, if you visit Course categories the images will load and stay in cache, then during subsequent visits they will appear before login. Course description images need to be located with Moodle homepage Site files I think, so they will display correctly to a new visitor.