When your publication uses its own HTML files, that are expecting to receive some additional resources (images, etc.), it is necessary to tell this to the editor. Otherwise, only the images (and other sources) that were used in the article will be sent to the distribution server.
Click the article setting icon and go to sources folder (Sources). Now click the button (Manage obligatory sources). Now you can click to add more images, videos etc. to the list of required sources, when you need to guarantee that they will be placed in the appropriate article folder on the distribution server.